MIPS: ralink: Add missing symbol for highmem support.

MT7621 has highmem. this was previously not working as the required symbol
was not selected in the Kconfig file.
Signed-off-by: John Crispin <john@phrozen.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14901/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/ralink/Kconfig

@@ -46,6 +46,7 @@ choice
 		select SYS_SUPPORTS_MULTITHREADING
 		select SYS_SUPPORTS_SMP
 		select SYS_SUPPORTS_MIPS_CPS
+		select SYS_SUPPORTS_HIGHMEM
 		select MIPS_GIC
 		select COMMON_CLK
 		select CLKSRC_MIPS_GIC