viafb: add clock source selection and PLL power management support

This patch adds some support for clock source selection as well as
PLL power management. The code is unused at the moment but was
successfully tested as far as possible.
The implementation is according to the documentation for VX700,
VX800, VX855, VX900. Probably the source selection works like this
starting with K800 and the power managemennt at least since VX700.
(guessed based on the initialization in viamode.c)

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>