target/riscv/cpu: set cpu->cfg in register_cpu_props()

There is an informal contract between the cpu_init() functions and
riscv_cpu_realize(): if cpu->env.misa_ext is zero, assume that the
default settings were loaded via register_cpu_props() and do validations
to set env.misa_ext. If it's not zero, skip this whole process and
assume that the board somehow did everything.

At this moment, all SiFive CPUs are setting a non-zero misa_ext during
their cpu_init() and skipping a good chunk of riscv_cpu_realize().  This
causes problems when the code being skipped in riscv_cpu_realize()
contains fixes or assumptions that affects all CPUs, meaning that SiFive
CPUs are missing out.

To allow this code to not be skipped anymore, all the cpu->cfg.ext_*
attributes needs to be set during cpu_init() time. At this moment this
is being done in register_cpu_props(). The SiFive boards are setting
their own extensions during cpu_init() though, meaning that they don't
want all the defaults from register_cpu_props().

Let's move the contract between *_cpu_init() and riscv_cpu_realize() to
register_cpu_props(). Inside this function we'll check if
cpu->env.misa_ext was set and, if that's the case, set all relevant
cpu->cfg.ext_* attributes, and only that. Leave the 'misa_ext' = 0 case
as is today, i.e. loading all the defaults from riscv_cpu_extensions[].

register_cpu_props() can then be called by all the cpu_init() functions,
including the SiFive ones. This will make all CPUs behave more in line
with what riscv_cpu_realize() expects.

This will also make the cpu_init() functions even more alike, but at this
moment we would need some design changes in how we're initializing
extensions/attributes (e.g. some CPUs are setting cfg options after
register_cpu_props(), so we can't simply add the function to a common
post_init() hook)  to make a common cpu_init() code across all CPUs.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230113175230.473975-2-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

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