dts: rp1: Add spi6, fix spi1 #address-cells

spi6 won't be useful on Pi 5 because it can't be enabled on the 40-pin
header, but include it for completeness.

Also fix the #address-cells value for spi1, otherwise the kernel will
reject attempts to apply the, say, spi1-2cs overlay at runtime.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>