omap4: pandaboard: ehci fref_clkout per board revision

omap4: pandaboard: ehci fref_clkout per board revision

Add support for correctly configuring the fref_clkout depending on the
board revision of the pandaboard. This patch is necessary to make u-boot
work with the smsc usb+ethernet driver.

Tested USB+Eth with TFTP and PXE using linaro u-boot:
http://git.linaro.org/gitweb?p=boot/u-boot-linaro-stable.git;a=summary

Based on patch from David Anders <x0132446@ti.com> from
omap4_panda_L24.9 branch at gitorious.org/pandaboard x-loader tree.

Changes since v1:
 * Moving the code from s_init instead of adding a new block

Signed-off-by: Ricardo Salveti de Araujo <ricardo.salveti@linaro.org>
Signed-off-by: Anand Gadiyar <gadiyar@ti.com>