spi: lpspi: Let watermark change with send data length

[ Upstream commit cf86874bb9bdb99ba3620428b59b0408fbc703d0 ]

Configure watermark to change with the length of the sent data.
Support LPSPI sending message shorter than tx/rxfifosize.

Signed-off-by: Clark Wang <xiaoning.wang@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Stable-dep-of: 730bbfaf7d48 ("spi: spi-fsl-lpspi: Fix scldiv calculation")
Signed-off-by: Sasha Levin <sashal@kernel.org>