spi: nxp-fspi: add octal mode flag bit for octal support

Add octal mode flags for octal I/O data transfer support.
NXP FlexSPI controller supports 8 lines Rx/Tx data transfer.

Signed-off-by: Yogesh Narayan Gaur <yogeshnarayan.gaur@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>