ATF can leave the MMC IP in a state where U-Boot mmc driver
can't enumerate the eMMC.

This patch provides a mmc0_reset_clk() function like we
already so do sd card controller which resets the IP
when entering U-Boot.

With this patch applied eMMC partitions are successfully
enumerated again.

=> mmc dev 0
switch to partitions #0, OK
mmc0(part 0) is current device
=> mmc part

Partition Map for MMC device 0  --   Partition Type: EFI

Part	Start LBA	End LBA		Name
	Attributes
	Type GUID
	Partition GUID
  1	0x00000800	0x00000fff	"vrl"
	attrs:	0x0000000000000000
	type:	ebd0a0a2-b9e5-4433-87c0-68b6b72699c7
	guid:	496847ab-56a1-4cd5-a1ad-47f4acf055c9
  2	0x00001000	0x000017ff	"vrl_backup"
	attrs:	0x0000000000000000
	type:	ebd0a0a2-b9e5-4433-87c0-68b6b72699c7
	guid:	61a36fc1-8efb-4899-84d8-b61642efa723
  3	0x00001800	0x00001fff	"mcuimage"
<snip>

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>