ARM: OMAP2+: hwmod: provide a function to return the address space of the MPU RT

A subsequent patch will need to know the struct omap_hwmod_addr_space
record corresponding to the module's register target, used by the MPU.
So, convert _find_mpu_rt_base() into _find_mpu_rt_addr_space().  Then
modify its sole current user, _populate_mpu_rt_base(), to extract the
MPU RT base address itself from the struct omap_hwmod_addr_space record.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Benoît Cousson <b-cousson@ti.com>