mmc: core: wait while adding MMC host to ensure root mounts successfully

MMC hosts are added asynchronously. We need to wait until detect returns to
avoid failed root filesystem mounts.

VFS: Cannot open root device "mmcblk0p1" or unknown-block(0,0): error -6
Please append a correct "root=" boot option; here are the available partitions:
mmc0: host does not support reading read-only switch. assuming write-enable.
1f00             256 mtdblock0  (driver?)
1f01             256 mtdblock1  (driver?)
1f02            2560 mtdblock2 mmc0: new SDHC card at address b368
 (driver?)
1f03           29696 mtdblock3  (driver?)
1f04           16384 mtdblock4 mmcblk0: mmc0:b368 USD   3.72 GiB
 (driver?)
 mmcblk0: p1
b300         3910656 mmcblk0  driver: mmcblk
  b301         3906560 mmcblk0p1 00000000-01
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)

Signed-off-by: Sergey Yanovich <ynvich@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>