[HIFN]: Improve PLL initialization

The current PLL initalization has a number of deficiencies:

- uses fixed multiplier of 8, which overclocks the chip when using a
  reference clock that operates at frequencies above 33MHz. According
  to a comment in the BSD source, this is true for the external clock
  on almost all every board.

- writes to a reserved bit

- doesn't follow the initialization procedure specified in chapter
  6.11.1 of the HIFN hardware users guide

- doesn't allow to use the PCI clock

This patch adds a module parameter to specify the reference clock
(pci or external) and its frequency and uses that to calculate the
optimum multiplier to reach the maximal speed. By default it uses
the external clock and assumes a speed of 66MHz, which effectively
halfs the frequency currently used.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Acked-by: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>