Add aarch64 psuedo help functions

Reduce code copy/paste by adding two helper functions for
aarch64_pseudo_read_value and aarch64_pseudo_write
The patch does not change any functionality.

2018-05-11  Alan Hayward  <alan.hayward@arm.com>

	* aarch64-tdep.c (aarch64_pseudo_read_value_2): New helper func.
	(aarch64_pseudo_write_2): Likewise.
	(aarch64_pseudo_read_value): Use helper.
	(aarch64_pseudo_write): Likewise.