MIPS: Malta: Add support for building MIPS R6 kernel

The Malta platform supports MIPS R6 (via QEMU or real bitstreams)
so add support for it.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>

arch/mips/Kconfig

@@ -377,8 +377,10 @@ config MIPS_MALTA
 	select SYS_HAS_CPU_MIPS32_R1
 	select SYS_HAS_CPU_MIPS32_R2
 	select SYS_HAS_CPU_MIPS32_R3_5
+	select SYS_HAS_CPU_MIPS32_R6
 	select SYS_HAS_CPU_MIPS64_R1
 	select SYS_HAS_CPU_MIPS64_R2
+	select SYS_HAS_CPU_MIPS64_R6
 	select SYS_HAS_CPU_NEVADA
 	select SYS_HAS_CPU_RM7000
 	select SYS_SUPPORTS_32BIT_KERNEL