arm64/sysreg: Move generation of RES0/RES1/UNKN to function
The RESx and UNKN define generation happens in two places
(EndSysreg and EndSysregFields), and was using nearly identical
code. Split this out into a function, and call that instead, rather
then keeping the dupliated code.
There are no changes to the generated sysregs as part of this change.