spi: bcm-qspi: Handle clock probe deferral

The clock provider may not be ready by the time spi-bcm-qspi gets
probed, handle probe deferral using devm_clk_get_optional().

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20200420190853.45614-2-kdasu.kdev@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>