pinctrl: add includes and ifdefs for non-DT builds

Commit e81c8f18
"pinctrl: pinconf-generic: add generic APIs for mapping pinctrl node"
Added function prototypes with implicit dependencies
on other header files causing build warnings like this:

In file included from
arch/arm/mach-ux500/board-mop500-pins.c:12:0:
include/linux/pinctrl/pinconf-generic.h:142:3:
warning: 'struct device_node' declared inside parameter list [enabled
by default]
   unsigned *reserved_maps, unsigned *num_maps);
   ^
include/linux/pinctrl/pinconf-generic.h:142:3:
warning: its scope is only this definition or declaration, which is
probably not what you want [enabled by default]
include/linux/pinctrl/pinconf-generic.h:142:3:
warning: 'struct pinctrl_dev' declared inside parameter list [enabled
by default]
include/linux/pinctrl/pinconf-generic.h:145:3:
warning: 'struct device_node' declared inside parameter list [enabled
by default]
   unsigned *num_maps);
   ^
Let's just add ifdefs for non-DT systems (the actual code is
already ifdefed) and #include <linux/device.h> to get the
most important structs and forward-declare the pinctrl
core structs.

Reported-by: Olof Johansson <olof@lixom.net>
Cc: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>