MIPS: loongson32: Remove regs-clk.h

Since commit c4649611 ("clk: loongson1: Remove
the outdated driver"), no one is using regs-clk.h.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>