ARM: kprobes: Reject probing MRS instructions which read SPSR

We need to reject probing of instructions which read SPSR because
we can't handle this as the value in SPSR is lost when the exception
handler for the probe breakpoint first runs.

This patch also fixes the bitmask for MRS instructions decoding to
include checking bits 5-7.

Signed-off-by: Jon Medhurst <tixy@yxit.co.uk>
Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>