nios2: fix wrong access to "SP" register in user_pt_regs

nios2: fix wrong access to "SP" register in user_pt_regs

Starting with linux commit v4.0-rc4~21^2, "SP" register
is available via regs[PTR_SP] member of struct user_pt_regs.

* linux/nios2/arch_regs.h (nios2_sp_ptr): Change type
to "unsigned int *".
* linux/nios2/arch_regs.c (nios2_sp_ptr): Likewise.
Replace .sp with .regs[PTR_SP].

Fixes: http://autobuild.buildroot.net/results/b9f/b9fc25b82f3280872fe1593ac252a8529ba83576
Fixes: v4.16-34-g6117728a ("nios2: export nios2_sp_ptr")
Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>