MIPS: loongson32: Remove regs-rtc.h

Since commit 35508d24 ("MIPS: loongson32: ls1c: Fix hang
during startup"), no one is calling ls1x_rtc_set_extclk().
Therefore, remove this obsolete function.

Since commit 9fb23090 ("rtc: Remove the Loongson-1
RTC driver"), no one is using regs-rtc.h.
Therefore, remove this obsolete header file.

Signed-off-by: Keguang Zhang <keguang.zhang@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>