bus: omap_l3_noc: improve readability by using helper for slave event parsing

Current interrupt handler does the first level parse to identify the
slave and then handles the slave even identification, reporting and
clearing of event as well. It is hence logical to split the handler
into two where the primary handler just parses the flagmux till it
identifies a slave and the slave handling, reporting and clearing is
done in a helper function.

While at it update the documentation in kerneldoc style.

Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Tested-by: Darren Etheridge <detheridge@ti.com>
Tested-by: Sekhar Nori <nsekhar@ti.com>