siimage: do clocking register posting earlier in setup_mmio_siimage()

Do clocking register posting earlier in setup_mmio_siimage()
to match code in init_chipset_siimage().

This is a preparation for the next patch which merges PCI and MMIO
code paths together.

Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>