ARM: mvebu: prefix coprocessor operand with p

In every other instance where mrc is used the coprocessor operand
is prefix with p (e.g. p15). Use the p prefix in this case too.
This fixes a build issue when using LLVM's integrated assembler:
  arch/arm/mach-mvebu/coherency_ll.S:69:6: error: invalid operand for instruction
   mrc 15, 0, r3, cr0, cr0, 5
       ^
  arch/arm/mach-mvebu/pmsu_ll.S:19:6: error: invalid operand for instruction
   mrc 15, 0, r0, cr0, cr0, 5 @ get the CPU ID
       ^

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>