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: NAndrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>