arch/unicore32/kernel/clock.c: add readl() and writel() for 'PM_' macros

Add readl() and writel() for 'PM_' macros, just like another areas have
done within unicored32, or will cause compiling issue.

The related error (allmodconfig for unicored32):

    CC      arch/unicore32/kernel/clock.o
  arch/unicore32/kernel/clock.c: In function 'clk_set_rate':
  arch/unicore32/kernel/clock.c:182: warning: initialization makes integer from pointer without a cast
  arch/unicore32/kernel/clock.c:204: error: lvalue required as left operand of assignment
  arch/unicore32/kernel/clock.c:206: error: lvalue required as left operand of assignment
  arch/unicore32/kernel/clock.c:207: error: invalid operands to binary & (have 'void *' and 'long unsigned int')
  make[1]: *** [arch/unicore32/kernel/clock.o] Error 1
  make: *** [arch/unicore32/kernel] Error 2

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Acked-by: Xuetao Guan <gxt@mprc.pku.edu.cn>
Signed-off-by: Xuetao Guan <gxt@mprc.pku.edu.cn>