spi: bcm-qspi: Use fastbr setting to allow faster MSPI speeds

Setting MSPI_SPCR3.fastbr=1 allows using clock divider (SPBR) values of
1-7, while the default value prohibits these values and requires a minimum
clock divider value of 8.

Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Link: https://lore.kernel.org/r/20200420190853.45614-8-kdasu.kdev@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>