mach-imx: bootaux: add dcache flushing before enabling M4

This patch fixes the issue with broken bootaux command,
when M4 binary is loaded and data cache isn't flushed
before M4 core is enabled.

Reproducing:
> tftpboot ${loadaddr} ${board_name}/hello_world.bin
> cp.b ${loadaddr} 0x7F8000 $filesize
> bootaux 0x7F8000

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>