viafb: split clock and PLL code to an extra file

This patch is a huge move operation with some rename and introduces
an interface to still use the functions. This should be a step in
the right direction to reuse the code whenever possible but cleanly
separate code that differs on different platform and keeping the
complexity as low as possible.
pll_config was renamed to via_pll_config to keep the naming scheme.

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