arm64: smp: Mark IPI globals as __ro_after_init

Mark the three IPI-related globals in smp.c as "__ro_after_init" since
they are only ever set in set_smp_ipi_range(), which is marked
"__init". This is a better and more secure marking than the old
"__read_mostly".

Suggested-by: Stephen Boyd <swboyd@chromium.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Chen-Yu Tsai <wenst@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20230906090246.v13.7.I625d393afd71e1766ef73d3bfaac0b347a4afd19@changeid


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>