cmd: spl: annotate switch/case fallthrough

cmd: spl: annotate switch/case fallthrough

The argument parsing in the SPL configuration command uses an implicit
switch/case fallthrough when dealing with a different number of
arguments.

Add our "fallthrough;" statement-like macro before the respective labels
in the bootm code, to avoid a warning when GCC's -Wimplicit-fallthrough
warning option is enabled.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Tom Rini <trini@konsulko.com>