ARM: dts: stm32: Auto-detect second MAC on STM32MP15xx DH electronics DHCOM

Test whether this system is compatible with STM32MP15xx DHCOM SoM,
if so, test whether R292 pull up is populated on pin PC3, which is
an indication that the second MAC chip, KS8851-16MLL, is populated.
Use this information to patch 'status' DT property into the second
ethernet MAC DT node and enable/disable the MAC on systems where
the chip is/isn't populated respectively.

Use spl_perform_fixups() to patch the U-Boot proper DT from SPL and
ft_board_setup() to patch Linux DT from U-Boot proper. This way both
software components are configured the same way.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>