LPC55(S)1x USB1 SRAM Address

[ehntoo]

I've started bringing up a new custom board based on the LPC5514 using this crate. So far, most things appear to be pretty well compatible, but I did run into an issue with the USB SRAM - on the 551x chips, the USB SRAM is mapped at 0x20010000 rather than at 0x40100000 as in their bigger-brother LPC55S69 cousins:

From what I can tell, this would be the first change in the crate for another chip family member. In other crates I've used like the stm32 hal packages, these differences are to be handled with feature flags. Would a similar strategy be the best way forward here? I suspect we could detect this particular difference easily at runtime as well. I can put together a PR if that would be helpful. :)