OvmfPkg: Reduce PcdMaxVariableSize with secure boot to avoid assert

r14252 causes OVMF to crash if SECURE_BOOT_ENABLE is set,
because PcdMaxVariableSize is set to a larger value than
required. In other platforms, 0x2000 seems to be sufficient.

Reported-by: Gary Ching-Pang Lin <glin@suse.com>
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14423 6f19259b-4bc3-4df7-8a09-765794883524