I tried to add support for STM32G4

I'm sure I haven't done everything correctly but I tried my best to add support for STM32G4 series. Would be nice if @Arksine complete my attempt :) There was a warning for flash.c: src/stm32/flash.c: In function 'flash_get_page_size': src/stm32/flash.c:40:1: warning: control reaches end of non-void function [-Wreturn-type] } adding CONFIG_MACH_STM32G4 to line 37 of file flash.c stopped the warning from popping up but I'm not sure if 128 * 1024 is correct for this series. Additionally "Build Katapult deployment application (Do not build) --->" and "Application start offset ----" has no option in menuconfig, I'm not sure if other options are all correct neither. @Arksine Thank you very much for considering adding support for G4 series 🙏