ARM: keystone: fix compilation warning

Fix the following compilation warning:

  arch/arm/mach-keystone/keystone.c:74:2: warning: initialization from incompatible pointer type [enabled by default]
  arch/arm/mach-keystone/keystone.c:74:2: warning: (near initialization for ‘__mach_desc_KEYSTONE.restart’) [enabled by default]

Signed-off-by: Vincent Stehlé <vincent.stehle@freescale.com>
Cc: Robin Holt <holt@sgi.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: trivial@kernel.org