ram: k3-ddrss: Fix register name and explain its usage

The k3-ddrss driver wants to configure the DDRSS_V2A_CTL_REG to reflect
the maximum possible SDRAM of 2 GB for AM64x (instead of the register's
default that says 8 GB, which the AM64x DDR controller wouldn't support).

The offset 0x20 was correct, but the register name DDRSS_V2A_R1_MAT_REG
was that of the next register at offset 0x24.

Signed-off-by: Dominic Rath <rath@ibv-augsburg.net>