xen/arm: add start to struct bootcmdline

xen/arm: add start to struct bootcmdline

Add a new start address field to struct bootcmdline to easily match a
cmdline to the corresponding bootmodule. This is useful for debugging
(not actually needed for functionalities today, but could be.)

Instead of printing the index in the cmdline array, print the start
address of the corresponding bootmodule for each cmdline in
early_print_info.

Signed-off-by: Stefano Stabellini <stefanos@xilinx.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>