ARM: ux500: delete devices-common remnants

commit f1ff5b25
"ARM: ux500: Stop enabling GPIOs when not booting with Device Tree"
deleted devices-common.c, but did not delete the build
rule from Makefile (most likely due to compiling in
a dirty tree where the object file still existed so
linking would complete anyway).

Delete the makerule, the header file associated
with this file and the remaining reference to the header.

Cc: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>