MIPS: ralink: add missing symbol for RALINK_ILL_ACC

A driver was added in commit 5433acd8

 ("MIPS: ralink: add illegal access
driver") without the Kconfig section being added. Fix this by adding the symbol
to the Kconfig file.

Signed-off-by: John Crispin <blogic@openwrt.org>
Reported-by: Paul Bolle <pebolle@tiscali.nl>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9299/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>