Simplify regcache::xfer_part

Simplify regcache::xfer_part

Since xfer_part is already a class method, and only
{raw,cooked}_{read,write} are passed to it.  We can remove these two
arguments, but add a bool argument is_raw, indicating raw registers or
cooked registers are accessed.

gdb:

2017-10-17  Yao Qi  <yao.qi@linaro.org>

	* regcache.c (regcache::xfer_part): Remove parameters read and
	write, add parameter is_raw.  All callers are updated.