clk: lpc32xx: add HCLK PLL output configuration

This patch add the support to setup the HCLK PLL output
using the "assigned-clock-rates" parameter in the device tree.

If the option is not use, the clock setup by the kickstart
and/or bootloader remain unchanged.

The previous kernel version did not change the clock frequency
output setup by the kickstart and/or bootloader;
this version always setup the clock frequency output to 208MHz.

Signed-off-by: Sylvain Lemieux <slemieux@tycoint.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>