openrisc: remove CONFIG_SYMBOL_PREFIX

Remove the SYMBOL_PREFIX Kconfig symbol as it's empty anyway.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Jonas Bonn <jonas@southpole.se>
Cc: linux@lists.openrisc.net
Cc: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Jonas Bonn <jonas@southpole.se>