mtd: rawnand: stm32_fmc2: depends on ARCH_STM32 instead of MACH_STM32MP157

To be able to compile the driver on all STM32MP SOCs, we move the
"depends on" on ARCH_STM32.

Signed-off-by: Christophe Kerello <christophe.kerello@foss.st.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230324155105.826063-3-christophe.kerello@foss.st.com