ARM: at91: fix board-rm9200-dt after sys_timer conversion

After "ARM: delete struct sys_timer" board-rm9200-dt
fails compilation with the following error:

  CC      arch/arm/mach-at91/board-rm9200-dt.o
arch/arm/mach-at91/board-rm9200-dt.c:50:2: error: unknown field 'timer' specified in initializer
arch/arm/mach-at91/board-rm9200-dt.c:50:13: error: 'at91rm9200_timer' undeclared here (not in a function)
make[1]: *** [arch/arm/mach-at91/board-rm9200-dt.o] Error 1
make: *** [arch/arm/mach-at91] Error 2

This is a fall out from the timer conversion. Fix it by
converting board-rm9200-dt to use new timer init
function as well.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>