Remove regcache_descr::nr_raw_registers

Remove regcache_descr::nr_raw_registers

struct regcache_descr has fields nr_raw_registers and gdbarch, and
nr_raw_registers can be got via gdbarch_num_regs (gdbarch), so it looks
nr_raw_registers is redundant.  This patch removes it and adds a protected
method num_raw_registers.

gdb:

2017-11-02  Yao Qi  <yao.qi@linaro.org>

	* regcache.c (struct regcache_descr) <nr_raw_registers>: Remove.
	(init_regcache_descr): Use gdbarch_num_regs.
	(regcache::regcache): Likewise.
	(regcache::get_register_status): Likewise.
	(regcache::assert_raw_regnum): Likewise.
	(regcache::cooked_read): Likewise.
	(regcache::cooked_read_value): Likewise.
	(regcache::cooked_write): Likewise.
	(regcache::dump): Likewise.
	(regcache::num_raw_registers): New method.
	* regcache.h (class regcache) <num_raw_registers>: New.