target/ppc: Rename spr_tcg.h to spr_common.h

Initial intent for the spr_tcg header was to expose the spr_read|write
callbacks that are only used by TCG code. However, although these
routines are TCG-specific, the KVM code needs access to env->sprs
which creation is currently coupled to the callback registration.

We are probably not going to decouple SPR creation and TCG callback
registration any time soon, so let's rename the header to spr_common
to accomodate the register_*_sprs functions that will be moved out of
cpu_init.c in the following patches.

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20220216162426.1885923-24-farosas@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>