gpio: provide lockdep keys for nested/unnested irqchips

The helper function for adding a GPIO chip compiles in a lockdep
key for debugging, the same key is needed for nested chips as
well.

The macro construction is unreadable, replace this with two
static inlines instead.

The _gpiochip_irqchip_add prefixed function is not helpful,
rename it with gpiochip_irqchip_add_key() that tell us what the
function is actually doing.

Fixes: d245b3f9

 ("gpio: simplify adding threaded interrupts")
Cc: Roger Quadros <rogerq@ti.com>
Reported-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Reported-by: Roger Quadros <rogerq@ti.com>
Reported-by: Grygorii Strashko <grygorii.strashko@ti.com>
Tested-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Tested-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>