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

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>
Series-cc: Andy Yan <andy.yan@rock-chips.com>
Series-cc: Klaus Goger <klaus.goger@theobroma-systems.com>
Series-cc: Heiko Stuebner <hs@sntech.de>
Series-cc: sjg

Series-version: 5

Cover-letter:
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.

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