aspeed: Introduce a 'hw_strap1' machine attribute

aspeed: Introduce a 'hw_strap1' machine attribute

To change default behavior of a machine and boot from eMMC, future
changes will add a machine option to let the user configure the
boot-from-eMMC HW strapping bit. Add a new machine attribute first.

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>