target/riscv/cpu.c: do not skip misa logic in riscv_cpu_realize()

target/riscv/cpu.c: do not skip misa logic in riscv_cpu_realize()

All RISCV CPUs are setting cpu->cfg during their cpu_init() functions,
meaning that there's no reason to skip all the misa validation and setup
if misa_ext was set beforehand - especially since we're setting an
updated value in set_misa() in the end.

Put this code chunk into a new riscv_cpu_validate_set_extensions()
helper and always execute it regardless of what the board set in
env->misa_ext.

This will put more responsibility in how each board is going to init
their attributes and extensions if they're not using the defaults.
It'll also allow realize() to do its job looking only at the extensions
enabled per se, not corner cases that some CPUs might have, and we won't
have to change multiple code paths to fix or change how extensions work.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng@tinylab.org>
Message-Id: <20230113175230.473975-3-dbarboza@ventanamicro.com>
[ Changes by AF:
 - Rebase
]
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>