The back-to-bootrom implementation for Rockchip has always relied on
the stack-pointer being valid on entry, so there was little reason to
have this as an assembly implementation.

This provides a new C-only implementation of save_boot_params and
back_to_bootrom (relying on setjmp/longjmp) and removes the older
assembly-only implementation.

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