sh: Get rid of nmi_count()

nmi_count() is a historical leftover and SH is the only user. Replace it
with regular per cpu accessors.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20201113141732.844232404@linutronix.de