ARM: tegra: set CONFIG_SYS_MMC_MAX_DEVICE

ARM: tegra: set CONFIG_SYS_MMC_MAX_DEVICE

If CONFIG_API is ever to be enabled on Tegra, this define must be set,
since api/api_storage.c uses it.

A couple of annoyting things about CONFIG_SYS_MMC_MAX_DEVICE

1) It isn't documented in README. The same is true for a lot of similar
   defines used by api_storage.c.

2) It doesn't represent MAX_DEVICE but rather NUM_DEVICES, since the
   valid values are 0..n-1 not 0..n.

However, I this patch does not address those shortcomings.

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