ARM: 6330/1: perf: reword comments relating to perf_event_do_pending

This is purely a cosmetic change to the ARM perf backend because the current
comments about the relationship between NMIs, interrupt context and
perf_event_do_pending are misleading.

This patch updates the comments so that they reflect what the code
actually does (which is in line with other architectures).

Acked-by: Jamie Iles <jamie.iles@picochip.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>