e500,book3s: mfspr 259: Register mapped/aliased SPRG3 user read

This patch registers mfspr 259 for Book3S and e500 family cores
following this research:

mfspr 259 provides read-only mapped user access to SPRG3(SPR 275) according to:

- PowerISA 2.02, Book III (documents implementation starting with POWER4+ @ p20)
- IBM PowerPC 970MP RISC Microprocessor User's Manual v2.1, page 48
- Amit Singh: "Mac OS X Internals: A Systems Approach" on 970 and 970FX cores:
  He demonstrates mfspr 259 reading TLS data from Mac OS X on G5 on page 588
- NXP documents it in the Core Reference Manuals of: e500, e500mc and e5500
- getcpu() of the 32 & 64-bit Book3S Linux vDSOs use it to read the core number

mfspr 259 does not appear to be implemented in these cores according to:

- 74xx series: MPC7410/MPC7400 and MPC7450 RISC Microprocessor Reference Manuals
- 4xx series:  PPC440 Processor User's Manual, Revision 1.09 by AMCC
- 750 series:  IBM PowerPC 750CL RISC Microprocessor User's Manual
- e200 series: e200z4 Power Architectureâ Core Reference Manual

Implementation: gen_spr_usprg3() is called from init_proc_book3s_common()
(covers the 970 and POWER cores) and init_proc_e500() (covers the e500 family)
to register spr_read_ureg() in the same way which it already provides
the mapped SPR access for SPR_USPRG4-7 in gen_spr_usprgh() for cores
which have the same read-only mapped SPRG register access for SPRG4-7.

Verified using Linux by pinning a thread to a core and checking sched_getcpu()
using qemu-system-ppc64 -M pseries -cpu POWER8 using MTTCG on a x86_64 host.

Signed-off-by: Bernhard Kaindl <bernhard.kaindl@thalesgroup.com>
Reviewed-by: Stefan Resch <stefan.resch@thalesgroup.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
