arm: mark save_boot_params_ret as a function

arm: mark save_boot_params_ret as a function

As no '.type' was set for save_boot_params_ret in start.S, binutils
did not track whether it was emitted as A32 or T32.  By properly
marking save_boot_params_ret as a potential function entry, we can
make sure that the compiler will insert the appropriate instructions
for branching to save_boot_params_ret both for call-sites emitted as
A32 and T32.

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