clocksource: add eco32 timer

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
eco32: mm: bail do_page_fault() when in_interrupt()

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
eco32: entry: avoid using $26 in preemptable places

We use $26 as a kernel scratch register in exceptions,
this means it can't be used in any place where we can be
preemptied, i.e. when interrupts are enabled.

Also mask out IC in PSW before restoring context.

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>