arm64: asm: setup.h: export common variables

arm64: asm: setup.h: export common variables

When building the kernel with sparse enabled 'C=1' the following
warnings can be seen:

arch/arm64/kernel/setup.c:58:13: warning: symbol '__fdt_pointer' was not declared. Should it be static?
arch/arm64/kernel/setup.c:84:25: warning: symbol 'boot_args' was not declared. Should it be static?

Rework so the variables are exported, since these two variable are
created and used in setup.c, also used in head.S.

Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Link: https://lore.kernel.org/r/20211007195601.677474-1-anders.roxell@linaro.org
Signed-off-by: Will Deacon <will@kernel.org>