disk: support devices with HW partitions

Some device types (e.g. eMMC) have hardware-level partitions (for eMMC,
separate boot and user data partitions). This change allows the user to
specify the HW partition they wish to access when passing a device ID to
U-Boot Commands such as part, ls, load, ums, etc.

The syntax allows an optional ".$hwpartid" to be appended to the device
name string for those commands.

Existing syntax, for MMC device 0, default HW partition ID, SW partition
ID 1:

ls mmc 0:1 /

New syntax, for MMC device 0, HW partition ID 1 (boot0), SW partition
ID 2:

ls mmc 0.1:2 /

For my purposes, this is most useful for the ums (USB mass storage
gadget) command, but there's no reason not to allow the new syntax
globally.

This patch adds the core support infra-structure. The next patch will
provide the implementation for MMC.

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