ramips: Fix early memory calculation for certain MIPS platforms

ramips: Fix early memory calculation for certain MIPS platforms

Kernel upstream commit 67a3ba25aa95 ("MIPS: Fix incorrect mem=X@Y handling") introduced a new issue for rt288x where "PHYS_OFFSET" is 0x0 but the calculated "ramstart" is not. As the prerequisite of custom memory map has been removed, this results in the full memory range of 0x0 - 0x8000000 to be marked as reserved
for this platform.

This patch adds the originally intended prerequisite again.

Signed-off-by: Tobias Wolf <dev-NTEO@vplace.de>