clocksource/drivers/sh_cmt: Mark "renesas,cmt-48-gen2" deprecated

Document in the driver that "renesas,cmt-48-gen2" is deprecated, but
still supported for backward compatibility with old DTBs, 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>