mmc: report capacity for the selected partition

Enhance the MMC core to calculate the size of each MMC partition, and
update mmc->capacity whenever a partition is selected. This causes:

mmc dev 0 1 ; mmcinfo

... to report the size of the currently selected partition, rather than
always reporting the size of the user partition.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
env_mmc: allow negative CONFIG_ENV_OFFSET

A negative value of CONFIG_ENV_OFFSET is treated as a backwards offset
from the end of the eMMC device/partition, rather than a forwards offset
from the start.

This is useful when a single board may be stuffed with different eMMC
devices, each of which has a different capacity, and you always want the
environment to be stored at the very end of the device (or eMMC boot
partition for example).

One example of this case is NVIDIA's Ventana reference board.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
ARM: tegra: make use of negative ENV_OFFSET on NVIDIA boards

Use a negative value of CONFIG_ENV_OFFSET for all NVIDIA reference boards
that store the U-Boot environment in the 2nd eMMC boot partition. This
makes U-Boot agnostic to the size of the eMMC boot partition, which can
vary depending on which eMMC device was actually stuffed into the board.

Signed-off-by: Stephen Warren <swarren@nvidia.com>