PCI: faraday: Add clock handling

Add some optional clock handling to the Faraday FTPCI100.  We just get and
prepare+enable the clocks right now, if they exist.  We can add more
elaborate clock handling later.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
[bhelgaas: folded in "Make clocks compulsory" fix from
http://lkml.kernel.org/r/20170621085730.28804-1-linus.walleij@linaro.org
Also folded in the clock max/cur speed fixes from
http://lkml.kernel.org/r/20170621162651.25315-1-linus.walleij@linaro.org

]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>