flash constants

[flihp]

I'm working on a tool to calculate the FWID for the lpc55 & stm32 images here: oxidecomputer/hubtools#37, To do so for the lpc55 requires we know the page size for flash. lpc55_support has 512 hard coded in a number of places (mostly around the size of the C[F|M]PA region (one flash page) but it wouldn't take much to expose that value as a constant.

[labbott]

In hubris we have a crate that has exactly one item: the flash page size https://github.com/oxidecomputer/hubris/blob/master/lib/lpc55-rom-data/src/lib.rs . Might be nice if we could make this work with no_std so we could get rid of that (not a strict requirement here)