ARM: mach-shmobile: kzm9g: add PCF8757 gpio-key

This patch adds extra GPIO via PCF8757 chip,
and use it as gpio-key.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Tested-by: Simon Horman <horms@verge.net.au>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>