cmd: mmc: add mmc partconf read capability

This patch allows to show the EXT_CSD[179] partition_config
register info, just by specifying the dev param:

  U-Boot> mmc partconf 0
  EXT_CSD[179], PARTITION_CONFIG:
  BOOT_ACK: 0x0
  BOOT_PARTITION_ENABLE: 0x0
  PARTITION_ACCESS: 0x0

Signed-off-by: Angelo Dureghello <angelo@sysam.it>
Signed-off-by: Anatolij Gustschin <agust@denx.de>