OvmfPkg: raise max variable size (auth & non-auth) to 33KB for FD_SIZE_4MB

The "ConfirmSetOfLargeVariable" test case of the Secure Boot Logo Test
("Microsoft.UefiSecureBootLogo.Tests") suite in the Microsoft Hardware
Certification Kit sets a 32 KB large non-authenticated variable.

In the FD_SIZE_4MB build, our live varstore is now 256 KB big, so we can
accommodate this. Set both PcdMaxVariableSize and PcdMaxAuthVariableSize
to 0x8400 -- beyond DataSize=0x8000 from the HCK test, we need some room
for the variable name and attributes as well.

Cc: Gary Ching-Pang Lin <glin@suse.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>