ARM: dts: vfxxx: Fix dspi[01] spi-num-chipselects.

Per the Vybrid Reference Manual (section 3.8.6.1), dspi0 has 6 chip
select signals associated with it, while dspi1 has only 4.

Signed-off-by: Cory Tusar <cory.tusar@pid1solutions.com>
Acked-by: Stefan Agner <stefan@agner.ch>
Cc: <stable@vger.kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>