[ARM] Kirkwood: add support for newer SoC models

Add support to the Kirkwood port for newer device models and silicon
revisions.  Instead of looking at the DEVICE_ID register, the device
version is now determined by looking at the PCI-Express device ID and
revision registers, as it is done for orion5x, and this information
is used to determine the TCLK frequency, again, as it is done for
orion5x.

Signed-off-by: Ronen Shitrit <rshitrit@marvell.com>
Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>