ARM: pxa: fix DFI bus lockups on startup

After the conversion of pxa architecture to common clock framework, the
NAND clock can be disabled on startup if no nand driver claims it.

In this case, it happens that if the bootloader used the NAND and set
the DFI arbitration bit, the next access to a static memory controller
area, such as an ethernet card, the system bus will stall, and the core
will be stalled forever.

Fix this by clearing the DFI arbritration bit in pxa3xx startup. The bit
will be enabled the pxa3xx-nand driver on need anyway. The only left
requirement is that upon pxa3xx-nand removal, the bit should be cleared
before the clock is disabled.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>