Silicon/SynQuacer/PlatformDxe: defer device registration until EndOfDxe

EDK2 carries an interesting quirk which dates back to the EFI 1.02 days,
where each protocol that is installed onto a handle during the execution
of a DXE driver's entrypoint is connected immediately (in the UEFI driver
model sense) after the entry point returns.

This means that, depending on the order that these drivers happen to end
up being dispatched, we may enter the BDS phase with the device's driver
stack fully connected, even if the device in question is not being used
to boot the machine.

Given the substantial delays that this might incur, let's work around
this 'feature' by deferring the registration of the network and eMMC
controllers to an EndOfDxe event handler.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>