Actually add the mpc85xx_get_platform_clock() function.

Follow up r319935 by actually committing the mpc85xx_get_platform_clock()
function. This function was created to facilitate other development, and I
thought I had committed it earlier.

Some blocks depend on the platform clock rather than the system clock.
The System clock is derived from the platform clock as one-half the
platform clock. Rewrite mpc85xx_get_system_clock() to use the new
function.

Pointy-hat to: jhibbits