powerpc: Add an inline function to update POWER8 HID0

powerpc: Add an inline function to update POWER8 HID0

Section 3.7 of Version 1.2 of the Power8 Processor User's Manual
prescribes that updates to HID0 be preceded by a SYNC instruction and
followed by an ISYNC instruction (Page 91).

Create an inline function name update_power8_hid0() which follows this
recipe and invoke it from the static split core path.

Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Reviewed-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Tested-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>