hikey: increase delay after eMMC initialized
hikey: increase delay after eMMC initialized

Some eMMC chips require a longer delay. After testing
different chips, 20ms appears to work reliably.

Signed-off-by: Ryan Grachek <ea3cd978650417470535f3a4725b6b5042a6ab59@edited.us>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware