xilinx-ppc.h: Drop

With PowerPC 4xx support removed, we have removed xilinx-ppc support as
well, remove this file now.

Fixes 98f705c9 ("powerpc: remove 4xx support")
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>