MIPS: mipsregs: Set proper ISA level for virt extensions

c994a3ec ("MIPS: set mips32r5 for virt extensions") setted
some instructions in virt extensions to ISA level mips32r5.

However TLB related vz instructions was leftover, also this
shouldn't be done to a R5 or R6 kernel buid.

Reorg macros to set ISA level as needed when _ASM_SET_VIRT
is called.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>