clocksource: sh_mtu2: Rename struct sh_mtu2_priv to sh_mtu2_device

Channel data is private as well, rename priv to device to make the
distrinction between the core device and the channels clearer.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Wolfram Sang <wsa@sang-engineering.com>