mmc: enable partition switch function for emmc

For emmc, it may have up to 7 partitions: two boot partitions, one
user partition, one RPMB partition and four general purpose partitions.
(Refer to JESD84-A44.pdf/page 154)

As bootloader may need to read out or reflashing images on those

different partitions, it is better to enable the partition switch with
console command support.

Also for partition would be restore to user partition(part 0) when CMD0
is used, so change mmc_init routine to perform normal initialization
only once for each slot, unless use the rescan command to force init
again.

Signed-off-by: Lei Wen <leiwen@marvell.com>
Acked-by: Andy Fleming <afleming@freescale.com>