powerpc/pmac: Remove spurrious machine type test

pmac_declare_of_platform_devices() is already a machine initcall, thus
it won't be called on a non-powermac machine. Testing for chrp there
is pointless.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>