openrisc: update DTLB-miss handler last

The self-modifying code that updates the TLB handler at start-up has
a subtle ordering requirement:  the DTLB handler must be the last thing
changed.

What I was seeing was the following:

i)   The DTLB handler was updated
ii)  The following printk caused a TLB miss and the look-up resulted
     in the page containing itlb_vector (0xc0000a00) being bounced from
     the TLB.
iii) The subsequent access to itlb_vector caused a TLB miss and reload
     of the page containing itlb_vector from the page tables.
iv)  But this reload of the page in iii) was being done by the "new"
     DTLB-miss handler which resulted (correctly) in the page flags being
     set to read-only; the subsequent write-access to itlb_vector thus
     resulted in a page (access) fault.

This is easily remedied if we ensure that the boot-time DTLB-miss handler
continues running until the very last bit of self-modifying code has been
executed.  This patch should ensure that the very last thing updated is the
DTLB-handler itself.

Signed-off-by: Jonas Bonn <jonas@southpole.se>
Acked-by: Julius Baxter <juliusbaxter@gmail.com>
Tested-by: Sebastian Macke <sebastian@macke.de>