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 <[e-mail]>
Cc: [e-mail]
Patchwork: https://patchwork.linux-mips.org/patch/14901/
Signed-off-by: Ralf Baechle <[e-mail]>

arch/mips/ralink/Kconfig

···
 		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