ARM: mmp: remove unused RTC register definitions

Now that register definitions have been moved to the driver, regs-rtc.h is
no longer used and can be removed.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Eric Miao <eric.y.miao@gmail.com>
Cc: Haojian Zhuang <haojian.zhuang@gmail.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>