ARM: ux500: Fix build errors/warnings when MACH_UX500_DT is not set

When MACH_UX500_DT and all related Device Tree configurations are forced
off the warning and error below prevent the kernel from compiling. This
simple patch fixes both issues and allows for full build and boot of
ST-Ericsson's low-cost development board, Snowball.

Warnings fixed:
  arch/arm/mach-ux500/board-mop500.c:680:32: warning: ‘snowball_of_platform_devs’ defined but not used

Errors fixed:
  arch/arm/mach-ux500/timer.c: In function ‘ux500_timer_init’:
  arch/arm/mach-ux500/timer.c:66:3: error: implicit declaration of function ‘of_find_matching_node’
  arch/arm/mach-ux500/timer.c:66:6: warning: assignment makes pointer from integer without a cast

Cc: stable@vger.kernel.org
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>