hw/omap_l4.c: Add helper function omap_l4_region_base

hw/omap_l4.c: Add helper function omap_l4_region_base

Add helper function omap_l4_region_base() to return the base address
of a particular region of an L4 target agent.

Signed-off-by: Juha Riihimäki <juha.riihimaki@nokia.com>
[Riku Voipio: Fixes and restructuring patchset]
Signed-off-by: Riku Voipio <riku.voipio@iki.fi>
[Peter Maydell: More fixes and cleanups for upstream submission]
Signed-off-by:  Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andrzej Zaborowski <andrew.zaborowski@intel.com>