powerpc: Add prefixed instructions to instruction data type

For powerpc64, redefine the ppc_inst type so both word and prefixed
instructions can be represented. On powerpc32 the type will remain the
same. Update places which had assumed instructions to be 4 bytes long.

Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
Reviewed-by: Alistair Popple <alistair@popple.id.au>
[mpe: Rework the get_user_inst() macros to be parameterised, and don't
      assign to the dest if an error occurred. Use CONFIG_PPC64 not
      __powerpc64__ in a few places. Address other comments from
      Christophe. Fix some sparse complaints.]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200506034050.24806-24-jniethe5@gmail.com

arch/powerpc/include/asm/ppc-opcode.h

@@ -158,6 +158,9 @@
 /* VMX Vector Store Instructions */
 #define OP_31_XOP_STVX          231
 
+/* Prefixed Instructions */
+#define OP_PREFIX		1
+
 #define OP_31   31
 #define OP_LWZ  32
 #define OP_STFS 52