gpio: brcmstb: allow all instances to be wakeup sources

This commit allows a wakeup parent interrupt to be shared between
instances.

It also removes the redundant can_wake member of the private data
structure by using whether the parent_wake_irq has been defined to
indicate that the GPIO device can wake.

Fixes: 19a7b694 ("gpio: brcmstb: Add interrupt and wakeup source support")
Signed-off-by: Doug Berger <opendmb@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Gregory Fong <gregory.0xf0@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>