[AArch64][SVE 12/32] Make more use of bfd_boolean

[AArch64][SVE 12/32] Make more use of bfd_boolean

Following on from the previous patch, which converted the
aarch64_reg_parse_32_64 parameters to bfd_booleans, this one
does the same for parse_address_main and parse_address.
It also documents the parameters.

This isn't an attempt to convert the whole file to use bfd_booleans
more often.  It's simply trying to avoid inconsistencies with new
SVE parameters.

gas/
	* config/tc-aarch64.c (parse_address_main): Turn reloc and
	accept_reg_post_index into bfd_booleans.  Add commentary.
	(parse_address_reloc): Update accordingly.  Add commentary.
	(parse_address): Likewise.  Also change accept_reg_post_index
	into a bfd_boolean here.
	(parse_operands): Update calls accordingly.

Change-Id: Ia8f585035886401fe282377bb32c4192329896e4