spi_nxp_lpspi: Clarify configuration function

Clarify at the top of the common lpspi file what is the purpose of the
file to be clear to future developers that this file is not supposed to
make any assumption about a particular implementation of the zephyr API
using the LPSPI, because I imagine it could be very likely that more
lpspi implementation will be done in the future to make different
tradeoffs than the current two. Also the current two are different
enough that we should avoid making assumptions even if they currently
hold for both because they might not always, as things change.

We should disable interrupt events while configuring the LPSPI
regardless of implementation. The specific implementation should enable
the interrupts it needs on its own transceive implementation.

Also clarify and simplify some code in the configure function. Namely,
we no longer need to check if we are already configured to write to
registers because a recent commit made it so that we clock the
peripheral from the zephyr driver init instead of upon the MasterInit
call on the SDK. There is also a redundant CR write which I have removed.

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