imx: mx6: implement functions to boot auxiliary core

Implement arch_auxiliary_core_up and arch_auxiliary_core_check_up.

arch_auxiliary_core_check_up is used to check whether M4 is running
or not. arch_auxiliary_core_up is to boot M4 core, the m4 core will
use the pc and stack which is set in arch_auxiliary_core_up to set R15
and R13 register and boot.

Signed-off-by: Ye.Li <ye.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>