powerpc: Add set_mode hcall

This new hcall in POWER8 is used to set various resource mode registers.
eg. it can set address translation mode on interrupt (note: partition wide
scope)

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>