ARM: Add definition for monitor mode

<asm/ptrace.h> provides *_MODE definitions for the various processor
modes, but monitor mode was missing.

Add MON_MODE to avoid code using the hardcoded value.

Suggested-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
---
ARM maintainers: Please provide your ack, as this is a dependency for a
mach-shmobile patch.

v3:
  - No changes,

v2:
  - New.
1 file changed