ARM: OMAP2+: Remove custom init_irq for remaining boards

With SoC specific timers, board specific init_irq is
no longer needed. Earlier this was still needed to
initialize the gptimer12 on Beagle based boards.

Also convert board-h4.c to use omap2_init_irq accidentally
did not get converted earlier.

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