ARM: OMAP: PRCM: add support for chain interrupt handler

Introduce a chained interrupt handler mechanism for the PRCM
interrupt, so that individual PRCM event can cleanly be handled by
handlers in separate drivers. We do this by introducing PRCM event
names, which are then matched to the particular PRCM interrupt bit
depending on the specific OMAP SoC being used.

PRCM interrupts have two priority levels, high or normal. High priority
is needed for IO event handling, so that we can be sure that IO events
are processed before other events. This reduces latency for IO event
customers and also prevents incorrect ack sequence on OMAP3.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Kevin Hilman <khilman@ti.com>
Cc: Avinash.H.M <avinashhm@ti.com>
Cc: Benoit Cousson <b-cousson@ti.com>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Govindraj.R <govindraj.raja@ti.com>
Tested-by: Kevin Hilman <khilman@ti.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>
[paul@pwsan.com: drop some dead code; use SoC-specific pending IRQ
 detection; move code to prm_common.c; add lots of documentation;
 remove saved_mask; add OCP barrier on ISR exit; improved error
 handling; split out per-SoC initialization to a separate patch]
Signed-off-by: Paul Walmsley <paul@pwsan.com>