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>