system: add RISC-V CPU info
This patch adds the missing information about RISC-V architecture,
which has been supported by OpenWrt. Currently, LuCI shows "?" at the
field of Architecture, we add "RISC-V" with isa string parsed from
/proc/cpuinfo.
For example, the following platform generates "RISC-V (rv64imafdc)":
root@OpenWrt:/# cat /proc/cpuinfo
processor : 0
hart : 0
isa : rv64imafdc
mmu : sv48
mvendorid : 0x31e
marchid : 0x8000000000008a45
mimpid : 0x820
Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>