i.MX6Q: spl: Fix falcon to use dram_init_banksize

Memory dt node update introduced by spl_fixup_fdt() in below
commit was making DDR configuration in-appropriate
to boot falcon mode. Hence added dram_init_banksize for
explicit assignment of  proper base and size of DDR.

"boot: fdt: Perform arch_fixup_fdt() on the given device tree for falcon boot"
(sha1: 6e7585bb)

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>