spi: bcm-qspi: add support for 3-wire mode for half duplex transfer

This change configures the MSPI controller to use 3-wire interface when
a slave device devictree nodes indicates this via the optional property.

Signed-off-by: Kamal Dasu <kdasu@broadcom.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20211008203603.40915-4-kdasu.kdev@gmail.com


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