ARM: OMAP2+: Fix compillation error in mach-omap2/timer.c

prom_add_property() has been renamed to of_add_property()
This patch fixes the following comilation error:

arch/arm/mach-omap2/timer.c: In function ‘omap_get_timer_dt’:
arch/arm/mach-omap2/timer.c:178:3: error: implicit declaration of function ‘prom_add_property’ [-Werror=implicit-function-declaration]
cc1: some warnings being treated as errors
make[1]: *** [arch/arm/mach-omap2/timer.o] Error 1
make[1]: *** Waiting for unfinished jobs....

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Olof Johansson <olof@lixom.net>