tegra2: Don't use board pointer before it is set up

In board_init_f() the gd->bd pointer is not valid when dram_init() is called.
This only avoids dying because DRAM is at zero on Tegra2. The common ARM
routine sets up the banks in the same way anyway, so we can just remove this
code.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>