clocksource: sh_mtu2: Add memory base to sh_mtu2_channel structure

The channel memory base is channel-specific, add it to the channel
structure in preparation for support of multiple channels per device.

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