ARM: imx_v6_v7_defconfig: enable power driver of RN5T618 PMIC family

There is a driver now for the power supply and fuel gauge functionality
of that chip family, so enable them, since they are used by various
i.MX6 boards, especially ebook-readers.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>