rockchip: spl: RK3399: use boot0 hook to create space for SPL magic

The SPL binary needs to be prefixed with the boot magic ('RK33' for
the RK3399) on the Rockchip platform and starts execution of the
instruction word following immediately after this boot magic.

This poses a challenge for AArch64 (ARMv8) binaries, as the .text
section would need to start on the odd address, violating natural
alignment (and potentially triggering a fault for any code that
tries to access 64bit values embedded in the .text section).

A quick and easy fix is to have the .text section include the 'RK33'
magic and pad it with a boot0 hook to insert 4 bytes of padding at the
start of the section (with the intention of having mkimage overwrite
this padding with the appropriate boot magic). This avoids having to
modify the linker scripts or more complex logic in mkimage.

X-AffectedPlatforms: RK3399-Q7
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Klaus Goger <klaus.goger@theobroma-systems.com>