8064: ci-bridge-spi: allows spi transfers at 10.8Mhz

Default spi transfer speed did not change, but it is
now possible to choose speed up to 10.8 Mhz.

Change-Id: I86188f40cc2b135c434dddeef2e0aa3f8ea49790
Signed-off-by: Bar Weiner <bweiner@codeaurora.org>