arm64: entry: make NMI entry/exit functions static

arm64: entry: make NMI entry/exit functions static

Now that we only call arm64_enter_nmi() and arm64_exit_nmi() from within
entry-common.c, let's make these static to ensure this remains the case.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20210607094624.34689-19-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>