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>
Tested-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>