powerpc: Fix build break with PPC_EARLY_DEBUG_BOOTX=y

A kernel configured with PPC_EARLY_DEBUG_BOOTX=y but PPC_PMAC=n and
PPC_MAPLE=n will fail to link:

  btext.c:(.text+0x2d0fc): undefined reference to `.rmci_off'
  btext.c:(.text+0x2d214): undefined reference to `.rmci_on'

Fix it by making the build of rmci_on/off() depend on
PPC_EARLY_DEBUG_BOOTX, which also enable the only code that uses them.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/misc_64.S

@@ -246,8 +246,8 @@ _GLOBAL(__bswapdi2)
 	or	r3,r7,r9
 	blr
 
-#if defined(CONFIG_PPC_PMAC) || defined(CONFIG_PPC_MAPLE)
 
+#ifdef CONFIG_PPC_EARLY_DEBUG_BOOTX
 _GLOBAL(rmci_on)
 	sync
 	isync
@@ -277,6 +277,9 @@ _GLOBAL(rmci_off)
 	isync
 	sync
 	blr
+#endif /* CONFIG_PPC_EARLY_DEBUG_BOOTX */
+
+#if defined(CONFIG_PPC_PMAC) || defined(CONFIG_PPC_MAPLE)
 
 /*
  * Do an IO access in real mode