i2c: pasemi: Allow to configure bus frequency

Right now the bus frequency has always been hardcoded as
100 KHz with the specific reference clock used in the PASemi
PCI controllers. Make this configurable to prepare for the
platform driver.

Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sven Peter <sven@svenpeter.dev>
Acked-by: Olof Johansson <olof@lixom.net>
Tested-by: Christian Zigotzky <chzigotzky@xenosoft.de>
Signed-off-by: Wolfram Sang <wsa@kernel.org>