ARM: make reserving the gd register a make variable

Currently all ARM targets spell out that r8 needs to be a reserved
register, while using a common crt0.s. Move this to a common make
variable so it is not repeated (and can be easily changed)

cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
ARM,relocate: do not use r9

r9 is a platform-specific register in ARM EABI and not per
definition a general purpose register. Do not use it while
relocating so it can be used for gd.

cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
ARM: use r9 for gd

To be more EABI compliant and as a preparation for building
with clang, use the platform-specific r9 register for gd
instead of r8.

note: The FIQ is not updated since it is not used in u-boot,
and under discussion for the time being.

The following checkpatch warning is ignored:
WARNING: Use of volatile is usually wrong: see
Documentation/volatile-considered-harmful.txt

cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
README: update ARM register usage

Besides the change of this patchset it also updates the
README to reflect that GOT-generated relocations are no
longer supported on ARM.

cc: Albert ARIBAUD <albert.u.boot@aribaud.net>