include: configs: verdin-am62: drop unused sdram address

Drop unused macro. This was copied straight from the AM62x EVM but while
meant for a second region of DDR this is not even needed for the AM62x
EVM configurations and has meanwhile also been dropped there.

Note that on the Verdin AM62, we do auto-detect the amount of SDRAM.

While at it also update the comment noting that CFG_SYS_SDRAM_SIZE is
the maximum which is only used for such auto-detection.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>