ARM: OMAP2+: Fix build error when only ARCH_OMAP2/3 or 4 is selected

Otherwise we'll get undefined reference to `gic_of_init' or
undefined reference to `omap_intc_of_init'.

This was caused by commit fbf75da7
(ARM: OMAP2+: board-generic: Use of_irq_init API).

Signed-off-by: Tony Lindgren <tony@atomide.com>