clocksource: sh_mtu2: Tidy up Kconfig typo for MTU2

It should be "MTU2" instead of "TMU2"

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Wolfram Sang <wsa@sang-engineering.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>