Display the programmer names for the SPRs instead of "SPR"

ARMv7-M/v8-M expose a set of special-purpose registers to the programmer: CONTROL, BASEPRI, PRIMASK, and FAULTMASK. From assembly language, they're separate registers, and you access one at a time.

ADIv5 appears to expose them all in a single "SPR", and Humility prints them that way, e.g. from probe:

humility:          SPR => 0x0

This is confusing because it doesn't match the programmer model of the system. It would be nice to break this out:

    CONTROL => 0x0
    BASEPRI => 0x0
    PRIMASK => 0x0
    FAULTMASK => 0x0

etc

(I actually didn't realize until Friday that Humility even read back the CONTROL register because I had scanned output looking for its name, and didn't see it.)

registers does a slightly better job in that it prints decodes of the constituent bits of SPR:

  SPR = 0x00000000 <- 0000_0000_0000_0000_0000_0000_0000_0000
                            |||         |         |         |
                            |||         |         |         + PRIMASK = 0
                            |||         |         +---------- BASEPRI = 0x0
                            |||         +-------------------- FAULTMASK = 0
                            ||+------------------------------ CONTROL.nPRIV = 0
                            |+------------------------------- CONTROL.SPSEL = 0
                            +-------------------------------- CONTROL.FPCA = 0

However, here too it'd be better to present these as separate registers.

Start with the code paths behind the probe and registers commands, searching for the SPR label and its ARM register decoding. Done means both commands present CONTROL, BASEPRI, PRIMASK, and FAULTMASK as separate programmer-facing registers while preserving the relevant decoded values.