arm_smcc_1_0_*: Fix to work on purecap.

- Load a capability pointer from the stack for purecap.

- Store capability registers to the results structure.  Even though
  the current firmware only returns integer registers, storing the
  full capability is ok as the values will just be null-derived.
  However, this approach matches the current structure's layout and
  will work with any future firmware that may return capabilities.

sys/dev/psci/smccc_arm64.S

@@ -107,10 +107,10 @@ ENTRY(arm_smccc_\insn)
 #ifdef __CHERI_PURE_CAPABILITY__
 	restore_registers
 #endif
-	ldr	x4, [PTRN(sp)]
+	ldr	PTR(4), [PTRN(sp)]
 	cbz	x4, 1f
-	stp	x0, x1, [PTR(4), #16 * 0]
-	stp	x2, x3, [PTR(4), #16 * 1]
+	stp	PTR(0), PTR(1), [PTR(4), #(2 * PTR_WIDTH) * 0]
+	stp	PTR(2), PTR(3), [PTR(4), #(2 * PTR_WIDTH) * 1]
 1:	ret
 END(arm_smccc_\insn)
 .endm