aspeed: Introduce a AspeedSoCClass 'boot_from_emmc' handler

aspeed: Introduce a AspeedSoCClass 'boot_from_emmc' handler

Report support on the AST2600 SoC if the boot-from-eMMC HW strapping
bit is set at the board level. AST2700 also has support but it is not
yet ready in QEMU and others SoCs do not have support, so return false
always for these.

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>