powerpc: wii: Remove outdated comment about memory fixups

The workaround has been removed. What stays is just code to find the
memory hole so the BATs can be configured properly in the function below.

Fixes: 57deb8fe ("powerpc/wii: Don't rely on the reserved memory hack")
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>