target/ppc: Refactor sleep and its variants to use a common helper

With sleep, doze, nap, rvwinkle and stop instructions moved to
decodetree, the helper these instructions used : gen_helper_pminsn,
should now be renamed to gen_helper_PMINSN, abiding by the decodetree
standards. Do the same.
Also refactor their translation routine to use a common do_sleep routine
since they share majority of the code.

Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Glenn Miles <milesg@linux.ibm.com>
Reviewed-by: Amit Machhiwal <amachhiw@linux.ibm.com>
Signed-off-by: Chinmay Rath <rathc@linux.ibm.com>
Tested-by: Aniket Sahu <asahu1x@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20260827133010.278889-20-rathc@linux.ibm.com
Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>