mfd: Fix stmpe.c build when OF is not enabled

Fix build errors when CONFIG_OF is not enabled by including
<linux/of.h> (needs to be added in any case).
An alternative fix could be to make the driver depend on OF.

drivers/mfd/stmpe.c:1025:2: error: implicit declaration of function 'of_property_read_u32'
drivers/mfd/stmpe.c:1030:2: error: implicit declaration of function 'for_each_child_of_node'
drivers/mfd/stmpe.c:1030:36: error: expected ';' before '{' token

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>