spi: spi-fsl-lpspi: Read chip-select amount from hardware for i.MX93

PARAM.PCSNUM register on i.MX93 indicates the number of supported
(hw) chip-selects. LPSPI4 has 3 while others have only 2.
Still allow overwriting from DT.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Link: https://lore.kernel.org/r/20230717085934.409476-3-alexander.stein@ew.tq-group.com


Signed-off-by: Mark Brown <broonie@kernel.org>