mmc: provide a select_hwpart implementation for get_device()

This enables specifying which eMMC HW partition to target for any U-Boot
command that uses the generic get_partition() function to parse its
command-line arguments.

Acked-by: Pantelis Antoniou <panto@antoniou-consulting.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>