of/dynamic: Fix test for PPC_PSERIES

"IS_ENABLED(PPC_PSERIES)" always evaluates to false, as IS_ENABLED() is
supposed to be used with the full Kconfig symbol name, including the
"CONFIG_" prefix.

Add the missing "CONFIG_" prefix to fix this.

Fixes: a25095d4 ("of: Move dynamic node fixups out of powerpc and into common code")

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: stable@vger.kernel.org #+3.17
Signed-off-by: Grant Likely <grant.likely@linaro.org>