ppc4xx: Detect if the sysclk on Sequoia is 33 or 33.333 MHz

The AMCC Secquoia board has been changed in a new revision from using a
33.000 MHz clock to a 33.333 MHz system clock. A bit in the CPLD
indicates the difference. This patch reads that bit and uses the correct
clock speed for the board. This code is backward compatable will all
prior boards. All prior boards will be read as 33.000.

Signed-off-by: Jeffrey Mann <mannj@embeddedplanet.com>
Signed-off-by: Stefan Roese <sr@denx.de>