cgem: Add support for the SiFive FU740

Note that currently Linux's device tree uses the FU540's compatible
string, as does upstream U-Boot, but the U-Boot shipped with the board
based on an older patch series has the correct FU740 name. Thankfully
they are the same, at least as far as software is concerned.

Whilst here, fix a style(9) nit.

Reviewed by: philip, kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31034