clk: shmobile: mstp: Consider "zb_clk" suitable for power management

Currently the CPG/MSTP Clock Domain code looks for MSTP clocks to power
manage a device.

Unfortunately, on R-Mobile APE6 (r8a73a4) and SH-Mobile AG5 (sh73a0),
the Bus State Controller (BSC) is not power-managed by an MSTP clock,
but by a plain CPG clock (zb_clk).  Add a special case to handle this,
so the clock is properly managed, and devices connected to the BSC work
as expected.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>