MIPS: Loongson: Define rtc device on MC146818-equipped systems

This patch declare the rtc device present on systems with clock compatible
with the mc146818 and handled by rtc-cmos.  Introduce a new Kconfig entry
because there are some systems without rtc_cmos compatible clock.

Signed-off-by: Arnaud Patard <apatard@mandriva.com>
To: linux-mips@linux-mips.org
Cc: aba@not.so.argh.org
Patchwork: http://patchwork.linux-mips.org/patch/1320/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>