board: colibri_imx7: reserve DDR memory for Cortex-M4

i.MX 7's Cortex-M4 core can run from DDR and uses DDR memory for
the rpmsg communication. Both use cases need a fixed location of
memory reserved. For the rpmsg use case the reserved area needs
to be in sync with the kernel's hardcoded vring descriptor location.

Use the linux,usable-memory property to carve out 1MB of memory
in case the M4 core is running. Also make sure that the i.MX 7
specific rpmsg driver does not get loaded in case we do not carve
out memory.

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

arch/arm/include/asm/mach-imx/sys_proto.h

@@ -153,6 +153,8 @@ void init_src(void);
 void init_snvs(void);
 void imx_wdog_disable_powerdown(void);
 
+int arch_auxiliary_core_check_up(u32 core_id);
+
 int board_mmc_get_env_dev(int devno);
 
 int nxp_board_rev(void);