viafb: add VIA slapping capability

This patch introduces dummy functions to execute when we don't know
what we should do (due to missing documentation). They do nothing
but print a nice message in the log explaining the situation.
To trigger this message initial power management support is
activated which might save a bit energy by disabling PLL and clock
if no device is configured to use them.

Note: The message is only shown for the oldest IGPs CLE266 and K400
      as for the other platforms there are reasonable assumptions
      how it does (hopefully) work.

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