of: push struct boot_param_header and defines into powerpc

Now powerpc is the only user of struct boot_param_header and FDT defines,
so they can be moved into the powerpc architecture code.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: linuxppc-dev@lists.ozlabs.org
Tested-by: Grant Likely <grant.likely@linaro.org>
Tested-by: Stephen Chivers <schivers@csc.com>