target/ppc: update {get,set}_dfp{64,128}() helper functions to read/write DFP numbers correctly

Since commit ef96e3ae96 "target/ppc: move FP and VMX registers into aligned vsr
register array" FP registers are no longer stored consecutively in memory and so
the current method of combining FP register pairs into DFP numbers is incorrect.

Firstly update the definition of the dh_*_fprp defines in helper.h to reflect
that FP registers are now stored as part of an array of ppc_vsr_t elements
rather than plain uint64_t elements, and then introduce a new ppc_fprp_t type
which conceptually represents a DFP even-odd register pair to be consumed by the
DFP helper functions.

Finally update the new DFP {get,set}_dfp{64,128}() helper functions to convert
between DFP numbers and DFP even-odd register pairs correctly, making use of the
existing VsrD() macro to access the correct elements regardless of host endian.

Fixes: ef96e3ae96 "target/ppc: move FP and VMX registers into aligned vsr register array"
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20190926185801.11176-4-mark.cave-ayland@ilande.co.uk>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

target/ppc/helper.h

@@ -686,7 +686,7 @@ DEF_HELPER_3(store_601_batu, void, env, i32, tl)
 #endif
 
 #define dh_alias_fprp ptr
-#define dh_ctype_fprp uint64_t *
+#define dh_ctype_fprp ppc_fprp_t *
 #define dh_is_signed_fprp dh_is_signed_ptr
 
 DEF_HELPER_4(dadd, void, env, fprp, fprp, fprp)