configs: icorem6: Use imx6 cratch register for bootcount

SRAM address used for bootcount on exiting code is erasing
previous count value when system reset from Linux. So use
the dedicated imx6 scratch register, GPR2 to preserve the
contents even if the system reset from Linux.

Fixes: 4eb9aa39

 ("configs: imx6qdl_icore_mmc: Enable watchdog and bootcounter")
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Tested-by: Shyam Saini <shyam.saini@amarulasolutions.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>