pnv_phb4_pec.c: move pnv_pec_phb_offset() to pnv_phb4.c

pnv_phb4_pec.c: move pnv_pec_phb_offset() to pnv_phb4.c

The logic inside pnv_pec_phb_offset() will be useful in the next patch
to determine the stack that should contain a PHB4 device.

Move the function to pnv_phb4.c and make it public since there's no
pnv_phb4_pec.h header. While we're at it, add 'stack_index' as a
parameter and make the function return 'phb-id' directly. And rename it
to pnv_phb4_pec_get_phb_id() to be even clearer about the function
intent.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20220110143346.455901-3-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>