ARM: mohawk: allow building with MMU disabled

It is in principle possible to build an MMP kernel for
the mohawk CPU with the MMU code disabled, except for one
simple build error:

proc-mohawk.S:345: Error: invalid operands (*UND* and *UND* sections) for `|'
proc-mohawk.S:345: Error: invalid operands (*ABS* and *UND* sections) for `|'
proc-mohawk.S:345: Error: invalid operands (*UND* and *UND* sections) for `|'
proc-mohawk.S:345: Error: invalid operands (*UND* and *UND* sections) for `|'
proc-mohawk.S:345: Error: undefined symbol L_PTE_USER used as an immediate value

This patch changes the proc-mohawk code to do the same as the
other CPUs and not try to actually do anything for the
cpu_mohawk_set_pte_ext function, which won't be used anyway.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>