aspeed: Add boot-from-eMMC HW strapping bit to rainier-bmc machine This value is taken from a running Rainier machine. Signed-off-by: Cédric Le Goater <clg@kaod.org> Reviewed-by: Andrew Jeffery <andrew@codeconstruct.com.au> Tested-by: Andrew Jeffery <andrew@codeconstruct.com.au> Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>