cmd: pmic: annotate switch/case fallthrough

cmd: pmic: annotate switch/case fallthrough

The argument parsing code in the pmic command uses an implicit switch/case
fallthrough to handle the common part of having one or two arguments.

Add our "fallthrough;" statement-like macro before the second branch in
the parsing 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>