nds32: Fix bug in bitfield.h

There two bitfield bug for perfomance counter
in bitfield.h:

	PFM_CTL_offSEL1		21 --> 16
	PFM_CTL_offSEL2		27 --> 22

This commit fix it.

Signed-off-by: Nickhu <nickhu@andestech.com>
Acked-by: Greentime Hu <greentime@andestech.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>