clocksource/drivers/sh_cmt: Use 0x3f mask for SH_CMT_48BIT case

Always use 0x3f as channel mask for the SH_CMT_48BIT type of devices.
Once this patch is applied the "renesas,channels-mask" property will
be ignored by the driver for older devices matching SH_CMT_48BIT. In
the future when all CMT types store channel mask in the driver then
we will be able to deprecate and remove "renesas,channels-mask" from DTS.

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