clocksource: sh_mtu2: Set initcall level to subsys

The reason for this is to ensure that MTU2 is probed earlier
than with its previous initcall level, module init.

This came up as a problem with using CMT as a clock source kzm9g-reference
which does not make use of early timers or devices. In that scenario
initialisation of SDHI and MMCIF both stall on msleep() calls due to the
absence of a initialised clock source.

The purpose of this change is to keep the MTU2 code in sync with the CMT code
which has been modified in a similar manner..

Compile tested only using se7206_defconfig.
I do not believe I have any boards that support the MTU2.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>