EmbeddedPkg: introduce GPIO PPI

EmbeddedPkg: introduce GPIO PPI

Introduce a PPI counterpart of the existing 'embedded GPIO' protocol,
so we can manipulate GPIOs from PEI modules. This allows things like
setting the boot mode based on a DIP switch setting.

Note that the naming is slightly awkward, as there is nothing 'embedded'
about a GPIO, but given that the DXE protocol already resides here and
has the 'embedded' prefix, it makes sense to retain uniformity.

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