ARM: mach-shmobile: SH73A0 external Ethernet fix

Keep the ZB clock enabled on sh73a0 to allow the BSC
to access external peripherals hooked up to CS signals.

This is needed to unbreak Ethernet support on sh73a0 boards
such as AG5EVM and Kota2 together with the following patch:

 794d78fe

 drivers: sh: late disabling of clocks V2

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>