MIPS: Quit exposing Kconfig symbols in uapi headers.

The kernel's struct pt_regs has many fields conditional on various
Kconfig variables, we cannot be exporting this garbage to user-space.

Move the kernel's definition to asm/ptrace.h, and put a uapi only
version in uapi/asm/ptrace.h gated by #ifndef __KERNEL__

Signed-off-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5305/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>