ARM: OMAP4+: prminst: provide function to find prm_dev instance offset

PRM device instance can vary depending on SoC. We already handle the
same during reset of the device, However, this is also needed
for other logic instances. So, first abstract this out to a generic
function.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>