target/mips: Add CP0 PWField register

target/mips: Add CP0 PWField register

Add PWField register (CP0 Register 5, Select 6).

The PWField register configures hardware page table walking for TLB
refills.

This register is required for the hardware page walker feature. It
exists only if Config3 PW bit is set to 1. It contains following
fields:

MIPS64:
BDI  (37..32) - Base Directory index
GDI  (29..24) - Global Directory index
UDI  (23..18) - Upper Directory index
MDI  (17..12) - Middle Directory index
PTI  (11..6 ) - Page Table index
PTEI ( 5..0 ) - Page Table Entry shift

MIPS32:
GDW  (29..24) - Global Directory index
UDW  (23..18) - Upper Directory index
MDW  (17..12) - Middle Directory index
PTW  (11..6 ) - Page Table index
PTEW ( 5..0 ) - Page Table Entry shift

Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Signed-off-by: Yongbok Kim <yongbok.kim@mips.com>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>