arm: make save_boot_params_ret prototype visible for AArch64

arm: make save_boot_params_ret prototype visible for AArch64

The save_boot_params_ret() prototype (for those of us, that have a
valid SP on entry and can implement save_boot_params() in C), was
previously only defined for !defined(CONFIG_ARM64).

This moves the declaration to a common block to ensure the prototype
is available to everyone that might need it.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Andy Yan <andy.yan@rock-chips.com>