AArch64: Fix SME za register description

Peter Maydell and Vacha Bhavsar pointed out that we have an incorrect
description of the SME za register in the documentation.  It is currently
described as a vector of SVL x SVL bytes, but that is incorrect.

What we really have is a 2-dimensional array of bytes, with each dimension
having size SVL.

Change the documentation to reflect that.

Approved-By: Eli Zaretskii <eliz@gnu.org>