drivers: sdhc: SDHC_INIT_PRIORITY default change breaks boards with SDHC depending on fixed regulators

**Describe the bug** Commit 713cdff1183 ("drivers: sdhc: set SDHC_INIT_PRIORITY to KERNEL_INIT_PRIORITY_DEVICE") changed the default of `SDHC_INIT_PRIORITY` from `85` to `KERNEL_INIT_PRIORITY_DEVICE` (50). `REGULATOR_FIXED_INIT_PRIORITY` defaults to `75`, so an SDHC device now initializes **before** the fixed regulators it depends on via devicetree (`vmmc-supply` / `vqmmc-supply`). This violates the devicetree dependency ordering enforced by `scripts/build/check_init_priorities.py`, and the build fails. Before that commit, SDHC (85) initialized after fixed regulators (75), which satisfied the dependency. **To Reproduce** Steps to reproduce the behavior: 1. `west build -p -b rcar_salvator_xs/r8a77961 samples/subsys/fs/littlefs -- -DCONF_FILE=prj_blk.conf` 2. Build fails at the device initialization priority validation step. Same failure with `-b rcar_h3ulcb/r8a77951/a57` (twister scenario `sample.filesystem.littlefs.blk`). **Expected behavior** The build succeeds, with the SDHC device initialized after its regulator dependencies. **Impact** Build failure (annoyance / CI breakage for affected boards). Any board whose SDHC/MMC node references fixed regulators is potentially affected, not only the two rcar boards above. **Logs and console output** ``` ERROR: Device initialization priority validation failed, the sequence of initialization calls does not match the devicetree dependencies. ERROR: /soc/mmc@ee140000 <rcar_mmc_init> is initialized before its dependency /regulator_1p8v <regulator_fixed_init> (POST_KERNEL+1 < POST_KERNEL+2) ERROR: /soc/mmc@ee140000 <rcar_mmc_init> is initialized before its dependency /regulator_3p3v <regulator_fixed_init> (POST_KERNEL+1 < POST_KERNEL+3) ninja: build stopped: subcommand failed. ``` Relevant `.config` values: ``` CONFIG_KERNEL_INIT_PRIORITY_DEVICE=50 CONFIG_REGULATOR_FIXED_INIT_PRIORITY=75 CONFIG_SDHC_INIT_PRIORITY=50 ``` **Environment (please complete the following information):** - OS: Linux - Toolchain: Zephyr SDK (aarch64-zephyr-elf) - Commit SHA or Version used: 713cdff1183 (current `main` HEAD) **Additional context** Possible fix directions: - Revert the default to a value that keeps SDHC after fixed regulators (e.g. `85`, or any value `> REGULATOR_FIXED_INIT_PRIORITY`), or - Have boards/SoCs with SDHC-to-regulator dependencies override `SDHC_INIT_PRIORITY` explicitly. The first option seems safer since the dependency pattern (SDHC supplied by fixed regulators) is not rcar-specific.