ARM: OMAP4: PM: Use custom omap_do_wfi() for default idle.

Default arch_idle() isn't good enough for OMAP4 because of aync bridge errata
and necessity of NOPs post WFI to avoid speculative prefetch aborts.
Hence Use OMAP4 custom omap_do_wfi() hook for default idle.

Later in the series, async bridge errata work-around patch updates the
omap_do_wfi() with necessary interconnects barriers.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Jean Pihet <j-pihet@ti.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>
Tested-by: Vishwanath BS <vishwanath.bs@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>