spi_nxp_lpspi: Remove MCUX branding

Since the LPSPI drivers no long use MCUX at all, remove the MCUX
branding, to avoid confusion. In the future if an implementation uses
the MCUX SDK driver, it should specifically be called by MCUX in the
name.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>