powernv:stop: Rename pnv_arch300_idle_init to pnv_power9_idle_init

powernv:stop: Rename pnv_arch300_idle_init to pnv_power9_idle_init

BugLink: http://bugs.launchpad.net/bugs/1666197
Balbir pointed out that the name of the function pnv_arch300_idle_init
was inconsistent with the names of the variables and functions
pertaining to POWER9 features in book3s_idle.S.

This patch renames pnv_arch300_idle_init to pnv_power9_idle_init.

This patch does not change any behaviour.

Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
(cherry picked from linux-next commit dd34c74c97b6c3ed1ac7caec0b46267142659aff)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>