[M68k][NFC] Rename M68kOperand::Kind to KindTy

Rename the M68kOperand::Type enumeration to KindTy to avoid ambiguity
with the Kind field when referencing enumeration values e.g.
`Kind::Value`.

This works around a compilation error under GCC 5, where GCC won't
lookup enum class values if you have a similarly named field
(see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60994).

The error in question is:
`M68kAsmParser.cpp:857:8: error: 'Kind' is not a class, namespace, or enumeration`

Differential Revision: https://reviews.llvm.org/D108723