ARM: mach-omap2: board-generic: fix undefined symbol

Since dra7 reuses the  function 'omap5_realtime_timer_init' in
arch/arm/mach-omap2/board-generic.c as timer init function, it has to be
built for this SoC as well.

Signed-off-by: Simon Barth <Simon.Pe.Barth@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>