gcc-plugins: update architecture list in documentation

Commit 65c059bc ("powerpc: Enable support for GCC plugins") enabled GCC
plugins on powerpc, but neglected to update the architecture list in the
docs. Rectify this.

Fixes: 65c059bc

 ("powerpc: Enable support for GCC plugins")
Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>