bus: ti-sysc: Drop MMU quirks

The OMAP IOMMU driver no longer uses the pm_runtime_irq_safe() API
after commit 1432ebbd ("iommu/omap: remove pm_runtime_irq_safe
flag for OMAP IOMMUs"), so the SYSC_QUIRK_LEGACY_IDLE quirk is no
longer needed for MMU devices. Drop the same.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>