arm64: Unconditionally support {ARCH_}HAVE_NMI{_SAFE_CMPXCHG}

From what I can see there isn't anything about ACPI_APEI_SEA that
means the arm64 architecture can or cannot support NMI safe
cmpxchg or NMIs, so the 'if' condition here is not important.
Let's remove it. Doing that allows us to support ftrace
histograms via CONFIG_HIST_TRIGGERS that depends on the arch
having the ARCH_HAVE_NMI_SAFE_CMPXCHG config selected.

Cc: Tyler Baicar <tbaicar@codeaurora.org>
Cc: Jonathan (Zhixiong) Zhang <zjzhang@codeaurora.org>
Cc: Dongjiu Geng <gengdongjiu@huawei.com>
Acked-by: James Morse <james.morse@arm.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>