spi: pxa2xx: choose closest lower speed

As per discussion [1] the best choice is to set closest speed which is not
going over the asked one.

Do the same approach for Intel Quark boards.

[1] http://www.spinics.net/lists/linux-spi/msg03389.html



Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>