fu540_prci: Rename to sifive_prci and use ocd_data for FU540 specificity

The FU740 has a very similar controller and will reuse most of the
driver. This also drops the dependency on the device-tree include for
the binding indices; the header doesn't namespace its contents (and nor
does the FU740 one) so using both would require seperate translation
units which would be unnecessarily complicated just to avoid defining
local copies of the small number of constants.

Whilst here, add the missing l to gemgxlclk's name and drop the prci_
prefix from tlclk's name as we don't prefix any of the others and it's
entirely unnecessary.

Reviewed by: kp, mhorne
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31021