ARM: dts: r8a7790: Remove CMT renesas,channels-mask

Update the DTS to remove the now deprecated "renesas,channels-mask"
property.

The channel information is now kept in the device driver and can easily
be determined based on the compat string.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>