OMAP4: DVFS: Fix scaling dependent frequencies.

When scaling dependent frequencies down it is observed that the order
in which frequencies are changed is incorrect.  For example, when core
domain changes OPP down from OPP119 to OPP50 the order is:
a) scale L3 freq down to OPP50
b) scale SGX freq down to OPP50
which is wrong.

Frequencies are scaled in order devices are registered in omap dvfs
framework in device list. Devices are inserted to the list in order,
such as dependences are satisfied if scaling frequencies up.

When scaling frequencies down traverse list backwards.

Change-Id: I752bcb264cac12514f358986a906f29ee81ba75e
Signed-off-by: Oleksandr Kozaruk <oleksandr.kozaruk@ti.com>