OvmfPkg: create a separate PlatformDebugLibIoPort instance for SEC

OvmfPkg: create a separate PlatformDebugLibIoPort instance for SEC

The next patch will want to add a global variable to
PlatformDebugLibIoPort, but this is not suitable for the SEC
phase, because SEC runs from read-only flash.  The solution is
to have two library instances, one for SEC and another
for all other firmware phases.  This patch adds the "plumbing"
for the SEC library instance, separating the INF files and
moving the constructor to a separate C source file.

Contributed-under: TianoCore Contribution Agreement 1.1
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen (Intel address) <jordan.l.justen@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>