target/ppc: Move isync instruction to decodetree

Moving the following instructions to decodetree specification :
isync

The changes were verified by validating that the tcg ops generated by
those instructions remain the same, which were captured with the '-d
in_asm,op' flag.

Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Nikhil Kumar Singh <nikhilks@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/20260827133010.278889-14-rathc@linux.ibm.com
Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>