clocksource: sh_mtu2: Drop incorrect SoC family name

The MTU2 hardware block is found in many Renesas SH and ARM SoCs, but
not in R-Car.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>