ARM: always perform per-CPU GIC init

The current code in ARMv8's lowlevel_init() skips the per-CPU GIC
initialization ifndef CONFIG_ARMV8_MULTIENTRY. However, the per-CPU init
should always occur; it's just the one-time init that should only happen
on the master.

Once this consideration is taken into account, the only difference between
the paths when CONFIG_ARMV8_MULTIENTRY is undefined/defined is the use of
branch_if_slave. Naively, any unified code would need to invoke this
conditionally upon ifdef CONFIG_ARMV8_MULTIENTRY. However, branch_if_slave
already checks CONFIG_ARMV8_MULTIENTRY and does nothing if it isn't
defined, so we don't even need that ifdef at the call site.

Reported-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>