clocksource: sh_mtu2: Split channel fields from sh_mtu2_priv

Create a new sh_mtu2_channel structure to hold the channel-specific
fields in preparation for multiple channels per device support.

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