aspeed/scu: Add boot-from-eMMC HW strapping bit for AST2600 SoC

aspeed/scu: Add boot-from-eMMC HW strapping bit for AST2600 SoC

Bit SCU500[2] of the AST2600 controls the boot device of the SoC.

Future changes will configure this bit to boot from eMMC disk images
specially built for this purpose.

Signed-off-by: Joel Stanley <joel@jms.id.au>
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>