EmbeddedPkg/Drivers: add virtual keyboard driver

EmbeddedPkg/Drivers: add virtual keyboard driver

This driver is used to simulate a keyboard. For example, user
could read GPIO setting or data from RAM address. If the value
matches the expected pattern, it could trigger a key pressed
event.

User needs to implement hooks of PLATFORM_VIRTUAL_KBD_PROTOCOL.
There're 4 hooks in this protocol.
Register(): Quote the interface that user needs. For example, user
needs to locate GPIO protocol if he wants to simulate a GPIO value
as a key.
Reset(): Do the initialization before reading value.
Query(): Read value. If the value matches the expected pattern,
trigger a key pressed event.
Clear(): Clean the value if necessary.

Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>