spi_qsd: Auto-select nearest lower available clock rate

Specifying max clock frequency for SPI, via platform data, requires
a frequency that is available to SPI. The frequencies that are
available to SPI and the devices max-frequency are specified in
different places and in different files. As a result theses values
can easily go out of sync.
SPI devices typically can operate at some maximum frequency, with
some tolerance for the actual-frequency being lower. SPI users
typically know their target-device's max-speed, but they have
to dig in platform data files to find the best matching frequency
that is available.
This patch lets users specify the device's max-frequency, while the
actual frequency is automatically selected by the driver. The selected
frequency is the same as the requested one, or the nearest lower one.

Change-Id: I293ec1b66ad56e4a2b4643482917f79dabd7a758
Signed-off-by: Gilad Avidov <gavidov@codeaurora.org>
Signed-off-by: Alok Chauhan <alokc@codeaurora.org>