[Blackfin] arch: fix bug - a crash on bootup with CONFIG_MPU on the BF548

The function flush_switched_dcplbs was clearing the CPLB entries covering
the process permission bitmasks.  This means that the sequence

	flush_switched_dcplbs ();
        set_mask_dcplbs(mm->context.page_rwx_mask);

has a problem: if kernel code (such as an interrupt) causes a CPLB miss before
set_mask_dcplbs completes, the CPLB handler function causes a double fault,
with an instantaneous reboot.

This bug fix is dedicated to Michael Hennerich, the only person in the world
capable of providing working JTAG hardware.

Signed-off-by: Bernd Schmidt <bernds_cb1@t-online.de>
Signed-off-by: Bryan Wu <cooloney@kernel.org>