target-mips: fix cpu_mips_validate_access()

type casting is required for 64 bit address

Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>
target-mips: update cpu definitions

P5600, MIPS32R6-generic, and I6400

Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>
Merge pull request #26 from yongbok/i6400-mips64r6-PRIP4

Update core configurations P5600, I6400 and mips32r6-generic.
Fix cpu_mips_validate_access()

Closes #23
Update for #17

Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>