target/ppc: Reduce code duplication across Power9/10 init code

target/ppc: Reduce code duplication across Power9/10 init code

Power9/10 initialization code consists of a lot of logical OR of
various flag bits as supported by respective Power platform during its
initialization, most of which is duplicated and only selected bits are
added or removed as needed with each new platform support being added.
Remove the duplicate code and share using common macros.

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
[adityag: renamed many POWERPC_* flags to PPC_* flags, checkpatch fixes]
Signed-off-by: Aditya Gupta <adityag@linux.ibm.com>
Tested-by: Amit Machhiwal <amachhiw@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>