clocksource: sh_cmt: Set initcall level to subsys

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

This came up as a problem with using 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.

Boot tested on: armadillo800eva, mackerel and kzm9g

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