i386/pc: Add e820 entry for SGX EPC section(s)

Note that SGX EPC is currently guaranteed to reside in a single
contiguous chunk of memory regardless of the number of EPC sections.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Message-Id: <20210719112136.57018-19-yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>