powerpc: Add config fragment for disabling -Werror

This makes it easy to disable building with -Werror:

  $ make defconfig
  $ grep WERROR .config
  # CONFIG_PPC_DISABLE_WERROR is not set
  CONFIG_PPC_WERROR=y

  $ make disable-werror.config
    GEN     Makefile
  Using .config as base
  Merging arch/powerpc/configs/disable-werror.config
  Value of CONFIG_PPC_DISABLE_WERROR is redefined by fragment arch/powerpc/configs/disable-werror.config:
  Previous value: # CONFIG_PPC_DISABLE_WERROR is not set
  New value: CONFIG_PPC_DISABLE_WERROR=y
  ...

  $ grep WERROR .config
  CONFIG_PPC_DISABLE_WERROR=y

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201023040002.3313371-1-mpe@ellerman.id.au