powerpc/of: Remove useless register save/restore when calling OF back

enter_prom() used to save and restore registers such as CTR, XER etc..
which are volatile, or SRR0,1... which we don't care about. This
removes a bunch of useless code and while at it turns an mtmsrd into
an MTMSRD macro which will be useful to Book3E.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/entry_64.S

@@ -823,30 +823,17 @@ _GLOBAL(enter_prom)
 	 * of all registers that it saves.  We therefore save those registers
 	 * PROM might touch to the stack.  (r0, r3-r13 are caller saved)
    	 */
-	SAVE_8GPRS(2, r1)
+	SAVE_GPR(2, r1)
 	SAVE_GPR(13, r1)
 	SAVE_8GPRS(14, r1)
 	SAVE_10GPRS(22, r1)
-	mfcr	r4
-	std	r4,_CCR(r1)
-	mfctr	r5
-	std	r5,_CTR(r1)
-	mfspr	r6,SPRN_XER
-	std	r6,_XER(r1)
-	mfdar	r7
-	std	r7,_DAR(r1)
-	mfdsisr	r8
-	std	r8,_DSISR(r1)
-	mfsrr0	r9
-	std	r9,_SRR0(r1)
-	mfsrr1	r10
-	std	r10,_SRR1(r1)
+	mfcr	r10
 	mfmsr	r11
+	std	r10,_CCR(r1)
 	std	r11,_MSR(r1)
 
 	/* Get the PROM entrypoint */
-	ld	r0,GPR4(r1)
-	mtlr	r0
+	mtlr	r4
 
 	/* Switch MSR to 32 bits mode
 	 */
@@ -860,8 +847,7 @@ _GLOBAL(enter_prom)
         mtmsrd  r11
         isync
 
-	/* Restore arguments & enter PROM here... */
-	ld	r3,GPR3(r1)
+	/* Enter PROM here... */
 	blrl
 
 	/* Just make sure that r1 top 32 bits didn't get
@@ -871,7 +857,7 @@ _GLOBAL(enter_prom)
 
 	/* Restore the MSR (back to 64 bits) */
 	ld	r0,_MSR(r1)
-	mtmsrd	r0
+	MTMSRD(r0)
         isync
 
 	/* Restore other registers */
@@ -881,18 +867,6 @@ _GLOBAL(enter_prom)
 	REST_10GPRS(22, r1)
 	ld	r4,_CCR(r1)
 	mtcr	r4
-	ld	r5,_CTR(r1)
-	mtctr	r5
-	ld	r6,_XER(r1)
-	mtspr	SPRN_XER,r6
-	ld	r7,_DAR(r1)
-	mtdar	r7
-	ld	r8,_DSISR(r1)
-	mtdsisr	r8
-	ld	r9,_SRR0(r1)
-	mtsrr0	r9
-	ld	r10,_SRR1(r1)
-	mtsrr1	r10
 	
         addi	r1,r1,PROM_FRAME_SIZE
 	ld	r0,16(r1)