mx7: psci: add basic psci support

1. add basic psci support for imx7 chip.
2. support cpu_on and cpu_off.
3. switch to non-secure mode when boot linux kernel.
4. set csu allow accessing all peripherial register in non-secure mode.

Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>