arm: imx: imx8m: Add basic PSCI provider implementation

arm: imx: imx8m: Add basic PSCI provider implementation

Implement basic PSCI provider to let OS turn CPU cores off and on,
power off and restart the system and determine PSCI version. This
is sufficient to remove the need for the ATF BL31 blob altogether.

To make use of this functionality, active the following Kconfig options:
  # CONFIG_PSCI_RESET is not set
  CONFIG_ARMV8_MULTIENTRY=y
  CONFIG_ARMV8_SET_SMPEN=y
  CONFIG_ARMV8_SPL_EXCEPTION_VECTORS=y
  CONFIG_ARMV8_EA_EL3_FIRST=y
  CONFIG_ARMV8_PSCI=y
  CONFIG_ARMV8_PSCI_CPUS_PER_CLUSTER=4
  CONFIG_ARMV8_SECURE_BASE=0x970000
  CONFIG_ARM_SMCCC=y
  CONFIG_SYS_HAS_ARMV8_SECURE_BASE=y

Signed-off-by: Marek Vasut <marex@denx.de>