greybus: spi: Separate out spilib from spi bridged PHY bundle driver

spilib can be used by multiple bridge drivers implementing different
bundle classes. Separate out bridged PHY bundle drivers parts.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>