Add omap_device_get_mpu_rt_va().  This is intended to be used by
device drivers (currently, via a struct platform_data function
pointer) to retrieve their corresponding device's virtual base address
that the MPU should use to access the device.  This is needed because
the omap_hwmod code does its own ioremap(), in order to gain access to
the module's OCP_SYSCONFIG register.

Add omap_hwmod_get_mpu_rt_va().  omap_device_get_mpu_rt_va() calls this
function to do the real work.

While here, rename struct omap_hwmod._rt_va to struct
omap_hwmod._mpu_rt_va, to reinforce that it refers to the MPU's
register target virtual address base (as opposed to, for example, the
L3's).

In the future, this belongs as a function in an omap_bus, so it is not
necessary to call this through a platform_data function pointer.

The use-case for this function was originally presented by Santosh
Shilimkar <santosh.shilimkar@ti.com>.
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>