tdesc_register_size returns number of bits, not bytes.
Rename to make it clearer.

Also, fixed bug in aarch64_get_tdesc_vq which assumed bytes.

gdb/
	* target-descriptions.c (tdesc_register_bitsize): Rename.
	* target-descriptions.h (tdesc_register_bitsize): Likewise.
	* rs6000-tdep.c (rs6000_gdbarch_init): Use new name.
	* aarch64-tdep.c (aarch64_get_tdesc_vq): Convert size.