[MIPS] Bigsur supports highmem.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Kconfig

@@ -546,6 +546,7 @@ config SIBYTE_BIGSUR
 	select SWAP_IO_SPACE
 	select SYS_HAS_CPU_SB1
 	select SYS_SUPPORTS_BIG_ENDIAN
+	select SYS_SUPPORTS_HIGHMEM
 	select SYS_SUPPORTS_LITTLE_ENDIAN
 
 config SNI_RM