The 'stack->phb_regs_mr' PHB4 passthrough XSCOM initialization relies on
'stack->phb' being not NULL. Moving 'stack->phb_regs_mr' region_init()
and add_subregion() to phb4_realize() time is a natural thing to do
since it's strictly PHB related.

The remaining XSCOM initialization is also related to 'stack->phb' but
in a different manner. For instance, 'stack->nest_regs_mr'
MemoryRegionOps, 'pnv_pec_stk_nest_xscom_ops', uses
pnv_pec_stk_nest_xscom_write() as a write callback. When trying to write
the PEC_NEST_STK_BAR_EN reg, pnv_pec_stk_update_map() is called. Inside
this function, pnv_phb4_update_regions() is called twice. This function
uses 'stack->phb' to manipulate memory regions of the phb.

This is not a problem now but, when enabling user creatable phb4s, a
stack that doesn't have an associated phb (i.e. stack->phb = NULL) it
will cause a SIGINT during boot in pnv_phb4_update_regions().

All this can be avoided if all XSCOM realize is moved to phb4_realize(),
when we have certainty about the existence of 'stack->phb'. A lot of
code was moved from pnv_phb4_pec.c to pnv_phb4.c due to static constant
and variables being used but the cleaner logic is worth the trouble.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20220111131027.599784-3-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>