ArmVirtPkg: set PcdMaxVariableSize and PcdMaxAuthVariableSize

ArmVirtPkg: set PcdMaxVariableSize and PcdMaxAuthVariableSize

To support UEFI Secure Boot and the Linux persistent store with UEFI
variables, set PcdMaxVariableSize to 0x2000 bytes as is done in OvmfPkg.
For reference, the related Ovmf commits: 8cee3de7 2d441ca9

Also increase the maximum size for Authenticated variables in order to
handle a larger Signature List size as is done in OvmfPkg. Related Ovmf
commit: f5404a3e

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Linn Crosetto <linn@hpe.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>