firmware: Add support for Qualcomm UEFI Secure Application

On platforms using the Qualcomm UEFI Secure Application (uefisecapp),
EFI variables cannot be accessed via the standard interface in EFI
runtime mode. The respective functions return EFI_UNSUPPORTED. On these
platforms, we instead need to talk to uefisecapp. This commit provides
support for this and registers the respective efivars operations to
access EFI variables from the kernel.

Communication with uefisecapp follows the standard Qualcomm Trusted
Environment (TEE or TrEE) / Secure OS conventions via the respective SCM
call interface. This is also the reason why variable access works
normally while boot services are active. During this time, said SCM
interface is managed by the boot services. When calling
ExitBootServices(), the ownership is transferred to the kernel.
Therefore, UEFI must not use that interface itself (as multiple parties
accessing this interface at the same time may lead to complications) and
cannot access variables for us.

Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>