spl: fdt: support for fdt fixup for falcon boot

Adding support for fdt fixup to update the
memory node in device tree for falcon boot.

This is needed for single stage or falcon
bootmode, to pass memory configuration to
kernel through DT memory node.

Signed-off-by: Ravi Babu <ravibabu@ti.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>