gpio: em: lock IRQs when starting them

This uses the new API for tagging GPIO lines as in use by
IRQs. This enforces a few semantic checks on how the underlying
GPIO line is used.

Also assign the gpio_chip.dev pointer to be used for error
messages.

ChangeLog v1->v2:
- Satisfy implicit semantics by calling .enable and .disable
  callbacks in the startup/shutdown callbacks.

Cc: Ian Molton <ian.molton@codethink.co.uk>
Cc: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>