clocksource/drivers/sh_cmt: Remove unused "renesas, channels-mask" handling

The in-driver channel configuration in sh_cmt_info.channels_mask is now
always set for all CMT devices instantiated from DT.

Hence the "renesas,channels-mask" property is no longer checked, and its
handling can be removed, cfr. commit 4e18111f ("devicetree:
bindings: Remove deprecated properties").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>