docs(prerequisites): update Arm compilers download link

docs(prerequisites): update Arm compilers download link

Right now, TF-A documentation recommends downloading Arm compilers
from:

  https://developer.arm.com/open-source/gnu-toolchain/gnu-a/downloads

However, this page is now deprecated, as indicated by the banner at
the top of the page. When navigating to the new recommended page, one
can see the following note, which provides the rationale for the
deprecation:

  GNU Toolchain releases from Arm were published previously as two
  separate releases - one for A-profile and the other for R & M
  profiles (GNU Toolchain for A-profile processors and GNU Arm
  Embedded Toolchain).

  Arm GNU Toolchain releases unifies these two into a single release
  and the previous way of releases therefore have been
  discontinued. However, the previous releases will continue to be
  available for reference.

This patch updates the link to the new recommended place for compiler
downloads.

Change-Id: Iefdea3866a1af806a5db2d2288edbb63c543b8ee
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>