cmd: part: Add 'number' sub-command

This sub-command serves for getting the partition index from
partition name. Also it can be used to test the existence of specified
partition.

Use case:
For example, in most CI environments this U-Boot command for automatic
testing of Linux rootfs is used:

    => setenv bootpart 1:f

where 0xf is "userdata" partition. But the number of "userdata"
partition can be changed any time, when partition table is changed.

So it would be nice to get rid of that 0xf magic number and use
partition name instead, like this:

    => part number mmc 1 userdata part_num
    => setenv bootpart 1:${part_num}

Signed-off-by: Ruslan Trofymenko <ruslan.trofymenko@linaro.org>
Signed-off-by: Igor Opaniuk <igor.opaniuk@gmail.com>
Reviewed-by: Alistair Strachan <astrachan@google.com>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>