common: Pass the boot device into spl_boot_mode()

The SPL code already knows which boot device it calls the spl_boot_mode()
on, so pass that information into the function. This allows the code of
spl_boot_mode() avoid invoking spl_boot_device() again, but it also lets
board_boot_order() correctly alter the behavior of the boot process.

The later one is important, since in certain cases, it is desired that
spl_boot_device() return value be overriden using board_boot_order().

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Andreas Bießmann <andreas.devel@googlemail.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Andreas Bießmann <andreas@biessmann.org>
[add newly introduced zynq variant]
Signed-aff-by: Andreas Bießmann <andreas@biessmann.org>