ARM: 8929/1: use APSR_nzcv instead of r15 as mrc operand

LLVM's integrated assembler does not accept r15 as mrc operand.
  arch/arm/boot/compressed/head.S:1267:16: error: operand must be a register in range [r0, r14] or apsr_nzcv
  1: mrc p15, 0, r15, c7, c14, 3 @ test,clean,invalidate D cache
                 ^

Use APSR_nzcv instead of r15. The GNU assembler supports this
syntax since binutils 2.21 [0].

[0] https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=db472d6ff0f438a21b357249a9b48e4b74498076

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>