overo: Fix regression introduced in a6b541b0

U-Boot has been broken on Overo boards since commit
a6b541b0.

This is because the gd pointer is not set early enough anymore,
such that the i2c_set_bus_num in get_board_revision can safely
execute. This results in a console hang at SPL and the boot does
not proceed.

This piece of code is anyway necessary only for really old Overo
boards with revision numbers <= 2410 and not required for the newer
boards. For these older boards, u-boot v2014.10 still works fine.

Signed-off-by: Arun Bharadwaj <arun@gumstix.com>