target/ppc: cpu_init: Expose some SPR registration helpers

The following patches will move CPU-specific code into separate files,
so expose the most used SPR registration functions:

register_sdr1_sprs         | 22 callers
register_low_BATs          | 20 callers
register_non_embedded_sprs | 19 callers
register_high_BATs         | 10 callers
register_thrm_sprs         | 8 callers
register_usprgh_sprs       | 6 callers
register_6xx_7xx_soft_tlb  | only 3 callers, but it helps to
                             keep the soft TLB code consistent.

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20220216162426.1885923-25-farosas@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>