[riscv] Ignore riscv,isa property in favour of direct CSR testing

[riscv] Ignore riscv,isa property in favour of direct CSR testing

The riscv,isa devicetree property appears not to be fully populated on
some real-world systems.  For example, the Sipeed Lichee Pi 4A
(running the vendor U-Boot) reports itself as "rv64imafdcvsu", which
does not include the "zicntr" extension even though the time CSR is
present and functional.

Ignore the riscv,isa property and rely solely on CSR testing to
determine whether or not extensions are present.

Signed-off-by: Michael Brown <mcb30@ipxe.org>