bus: omap_l3_noc: switch over to relaxed variants of readl/writel

Currently we use __raw_readl and writel in this driver. Considering
there is no specific need for a memory barrier, replacing writel
with endian-neutral writel_relaxed and replacing __raw_readls with
the corresponding endian-neutral readl_relaxed allows us to have a
standard set of register operations for the driver.

While at it, simplify address computation using variables for
register.

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