riscv: cpufeature: add validation for zfa, zfh and zfhmin

riscv: cpufeature: add validation for zfa, zfh and zfhmin

These extensions depends on the F one. Add a validation callback
checking for the F extension to be present. Now that extensions are
correctly reported using the F/D presence, we can remove the
has_fpu() check in hwprobe_isa_ext0().

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20250527100001.33284-1-cleger@rivosinc.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>