mem-buf: Replace *_TRUSTED_UI with *_TRUSTED_VM

Trusted UI is simply a use-case that runs within the
trusted VM. The VM can have multiple usecases, and as
such, tying a usecase name to the VMID identifiers for the
VM is not appropriate. Synchronize with the rest of the kernel,
which refers to the trusted VM as TRUSTED_VM.

Change-Id: Ia54c3d4f36e8022f639164084a612b35e445209f
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>