clk: imx: increase AXI clock rate to 264MHz for i.MX6UL

On i.MX6UL, AXI clock rate's design target is 264MHz, but by default
it is only set to 198MHz which is NOT good enough for performance,
this patch increases AXI clock rate from 198MHz to 264MHz to meet
the design target, this is done by switching its parent clock
"periph" from 396MHz PFD to 528MHz PLL.

Signed-off-by: Anson Huang <b20788@freescale.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>