rockchip: back-to-bootrom: replace assembly-implementation with C-code

Recent discussions confirmed (what the code always assumed): the Rockchip BROM always enters U-Boot with the stack-pointer valid (i.e. the U-Boot startup code is running off the BROM stack).

We can thus replace the back-to-bootrom code (i.e. both the save_boot_params and back_to_bootrom implementations) using C-code based on setjmp/longjmp. The new implementation is already structured to allow an easy drop-in of Andy's changes to enter download-mode when returning to the BROM.

This turned out to require a some tweaking to system.h (making sure that the prototype for save_boot_params_ret is visible for A64)and start.S (so binutils knows that this is a possible function entry and it can correctly insert A32-to-Thumb transitions) and taking the axe to setjmp.h (which created quite a few issues with it not expecting A32/T32/Thumb call-sites and some fragility from GCC being smart about the clobber-list of the inline assembly... which led to r9 not being saved or restored).

For v4+: To fix issues with the RK3188 support, this also updates the boot0 hook changes (i.e. allowing the boot0-hook to insert code/data before the ARM vector table) that Kever had submitted this spring and implements a similar "early back-to-bootrom" as suggested by Pawel (for the RK3066) as a generic mechanism.