spi: npcm_pspi: use ACTIVE_LOW flag for cs gpio and set default max_hz

If cs gpio is requested with ACTIVE_HIGH flag, it will
be pulled low(i.e. active). This is not what we expected.

Signed-off-by: Jim Liu <JJLIU0@nuvoton.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>