sh: always enable sh7724 vpu_clk and set to 166MHz on Ecovec

Update the sh7724 processor code to always enable vpu_clk.

On the Ecovec board, set the vpu_clk to 166 Mhz.

The 166MHz setting results in a divide-by-6 setup for
vpu_clk and improves the VPU performance compared to the
power-on-reset/bootloader configuration.

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