[SCB] More useful definitions for SHPR fields

The direct access definitions to the SHPR1 SHPR2 and SHPR3 were simply
wrong, and though a handy macro for directly reaching any priority field
was provided, the definitions of the fields assumed direct access to the
registers.

Provide field definitions that match the handy macro, and correct the
direct access definitions for people wishing to use them.

(This is just an easier place for discussion than on my own branch and the mailing list)