spi_nxp_lpspi: Remove mcux branding from tokens

Since these drivers mainly do not use MCUX except for the configure
function (which will soon also be changed), change namespace prefix to
lpspi_ instead of spi_mcux_ to avoid confusion.

Also improve descriptions of kconfigs to clarify what they are for.
Not changing the kconfig names for now since they are user-facing.

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