arm: arm926ej-s: start.S: port save_boot_params support from armv7 code

The ARMv7 start code has support for saving some boot params at the
entry point, which is used by some SoCs to return to BROM.

Port this to ARM926EJ-S start code.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Jesse Taube <Mr.Bossman075@gmail.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>