ARM: perf: use u32 instead of unsigned long for PMNC register

The ARMv7 perf backend mixes up u32 and unsigned long, which is rather
ugly.

This patch makes the ARMv7 PMU code consistently use the u32 type
instead.

Acked-by: Jamie Iles <jamie@jamieiles.com>
Reviewed-by: Jean Pihet <j-pihet@ti.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>