arm: Switch aarch64 to using generic global_data setup

There is quite a bit of assembler code that can be removed if we use the
generic global_data setup. Less arch-specific code makes it easier to add
new features and maintain the start-up code.

Drop the unneeded code and adjust the hooks in board_f.c to cope.

Tested on LS2085ARDB and LS2085AQDS (armv8 SoC).
Tested-by: York Sun <yorksun@freescale.com>

Signed-off-by: Simon Glass <sjg@chromium.org>