msm: kgsl: Add kobject sysfs ops for the /sys/kernel/gpu directory

Commit 2726e792 ("msm: kgsl: Fix linked sysfs files") thought it was
clever by reusing the callbacks for the sysfs nodes that were linked to
/sys/kernel/gpu. Unfortunately, since the /sys/kernel/gpu kobject didn't
have a device attached the callback function ended up dereferencing
nothing.

Instead, create a full kobject infrastructure with sysfs_ops and a built
in kobj to access the KGSL device. We can still use the callbacks albeit
with a wrapper to make them work from both locations. While we are in
the midst of this, make a header for the new attributes and macros so
that we can later move some of the nodes out of kgsl_pwrctrl.c with
less disruption.

Change-Id: Ic0dedbad5b9d7323d4f43328540dce91cc58015e
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>