ARM: imx_v6_v7_defconfig: Enable the PSCI CPUidle driver

Enable the PSCI CPUidle driver to replace the functionality
previously provided by the generic ARM CPUidle driver.

Link: https://lore.kernel.org/r/20190814125239.6270-3-lorenzo.pieralisi@arm.com


Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>