doc: board: phytec: Document DDR size override Kconfigs

doc: board: phytec: Document DDR size override Kconfigs

The phyCORE-AM62x and phyCORE-AM64x R5 SPL detects the populated DDR
size from the SoM EEPROM and falls back to 2 GB if detection fails. For
boards without a populated EEPROM or if no detection needed, the detection
can be bypassed via CONFIG_PHYCORE_AM6{2,4}X_RAM_SIZE_FIX and one of
the CONFIG_PHYCORE_AM6{2,4}X_RAM_SIZE_<size> choices.

Add a "DDR RAM Size" section to both board docs describing this
behaviour and listing the available size options (1/2/4 GB for AM62x,
1/2 GB for AM64x).

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>