[c-nsp] 2gb on 720BXL w/ SXI

[Kevin Graham]

Stumbled across this when reading SXI release notes, which is the only mention I'd seen of it. As of SXI, 2gb of DRAM is supported on both RP and SP of Sup720BXL. Not sure what the motivation was to take SP up, but MSFC3 w/ 2gb takes some of the sting out of MSFC4 getting blocked on 6500...

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/release/notes/ol_14271.html#wp4148909