mmc: sdhci-pxav3: Try to get named I/O clock first

With support for more than one clock, we'll need to distinguish between
the clock by name. Change clock probing to first try to get "io" clock
before falling back to unnamed clock.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>