arm: imx: imx8m: imx9: Fix DRAM size calculation due to rom_pointer

arm: imx: imx8m: imx9: Fix DRAM size calculation due to rom_pointer

If dram_init_banksize() is called from SPL, the rom_pointer, at that
point, is not correctly initialized. This causes wrong calculation of
DRAM start and size in dram_init_banksize(). The issue became apparent
only in Falcon Mode. Added an extra condition to prevent using
rom_pointer in SPL.

Signed-off-by: Elena Popa <elena.popa@nxp.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Fabio Estevam <festevam@denx.de>