ARM: dts: ls1021a: DSPI has 6 chip-selects

Both DSPI have signals SPIn_PCS[0:5] so in summary 6 chip-selects, not 5.
Fix that.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>