tegra2: Move board_mmc_init into board files

For Seaboard, this is mostly a cut/paste of board_mmc_init() and
pin_mux_mmc() into seaboard.c; pin_mux_mmc() was modified to add some
missing pinmux_tristate_disable calls for the GPIOs.

For Harmony, those functions were modified to configure SDMMC2 (index 2)
instead of SDMMC3 (index 1), since that's what is present on the board.

However, harmony.c is still missing the required GPIO setup, so neither
port is likely to function correctly yet.  This will be fixed in the next
change.

v4: Include board.h to prototype tegra2_mmc_init().

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