x86/msr: Clean up the MSR_EFER constants

x86/msr: Clean up the MSR_EFER constants

There are no remaining users of the bit position constants.  Move the used
constants into the cleaned-up area of msr-index.h and apply appropriate style.

Rename EFER_NX to EFER_NXE to match both the Intel and AMD specs.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>