ANDROID: KVM: arm64: Add serial framework for pKVM

Debugging a hypervisor tends to be trickier than normal system code
such as the kernel. The lack of availability of a UART framework is a
significant contributor to that. In order to address this, introduce a
framework allowing to load serial drivers into the hypervisor.

Bug: 278749606
Bug: 244543039
Bug: 244373730
Change-Id: I2e7a1fd9abc9d5aa9d95f1d271a997d54a8fd582
Signed-off-by: Quentin Perret <qperret@google.com>