arm: mvebu: spl: Always fallback to BootROM boot method

BootROM boot method should always work so always add it as fallback method
to spl_boot_list. In case U-Boot SPI driver fails it is better to try using
BootROM than hanging as by default only one boot method is specified.

Signed-off-by: Pali Rohár <pali@kernel.org>
Tested-by: Tony Dinh <mibodhi@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>