davinci: Do not duplicate setting of gd

In f0c3a6c4 we stopped setting gd in board_init_f, but later had to
revert to due problems on certain platforms.  As davinci does not look
to have these problems, we can drop the setting here and rely upon
crt0.S to do it.

Cc: Peter Howard <pjh@northern-ridge.com.au>
Signed-off-by: Tom Rini <trini@ti.com>