arm64: dts: hikey: Enable HS200 mode on eMMC

According to the hi6220 datasheet, the MMC controller is JEDEC eMMC 4.5
compliant, in addition to supporting a clock of up to 150MHz. The Hikey
schematic also indicates the device utilizes 1.8v signaling. Define these
parameters in the device tree to enable HS200 mode.

Signed-off-by: Ryan Grachek <ryan@edited.us>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>