ARM: vfp: ensure preemption is disabled when enabling VFP access

The vfp_enable function enables access to the VFP co-processor register
space (cp10 and cp11) on the current CPU and must be called with
preemption disabled. Unfortunately, the vfp_init late initcall does not
disable preemption and can lead to an oops during boot if thread
migration occurs at the wrong time and we end up attempting to access
the FPSID on a CPU with VFP access disabled.

This patch fixes the initcall to call vfp_enable from a non-preemptible
context on each CPU and adds a BUG_ON(preemptible) to ensure that any
similar problems are easily spotted in the future.

originally from
http://git.kernel.org/?p=linux/kernel/git/will/linux.git;a=commit;h=468c963e0210bf8108b17cf75066f25f39cabb56

Change-Id: I26fff8abe4c18bd3291613f70d0228aa2313811a
Reported-by: Hyungwoo Yang <hwoo.yang@gmail.com>
Signed-off-by: Hyungwoo Yang <hyungwooy@nvidia.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Reviewed-on: http://git-master/r/102315
GVS: Gerrit_Virtual_Submit
Reviewed-by: Thomas Cherry <tcherry@nvidia.com>
Reviewed-by: Yu-Huan Hsu <yhsu@nvidia.com>

Conflicts:

	arch/arm/vfp/vfpmodule.c