cmd: mmc: Consider GP partitions in mmc hwpartition user enh start -

cmd: mmc: Consider GP partitions in mmc hwpartition user enh start -

In case the eMMC contains any GP partitions or user sets up new GP
partitions, the size of these GP partitions reduce the size of the
USER partition. Subtract the size of those GP partitions from the
calculated size of USER partition when using `user enh start -`.

The following test used to fail before:
```
u-boot=> mmc hwpartition gp1 524288 enh user enh 0 - wrrel on check
Partition configuration:
        User Enhanced Start: 0 Bytes
        User Enhanced Size: 1.8 GiB
        User partition write reliability: on
        GP1 Capacity: 256 MiB ENH
        No GP2 partition
        No GP3 partition
        No GP4 partition
Total enhanced size exceeds maximum (261 > 229)
Failed!
```
The test now passes:
```
u-boot=> mmc hwpartition gp1 524288 enh user enh 0 - wrrel on check
Partition configuration:
        User Enhanced Start: 0 Bytes
        User Enhanced Size: 1.5 GiB
        User partition write reliability: on
        GP1 Capacity: 256 MiB ENH
        No GP2 partition
        No GP3 partition
        No GP4 partition
```

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>