riscv: Add the T-HEAD SoC family Kconfig option

The first SoC in the T-HEAD series is TH1520, containing quad T-HEAD
C910 cores.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>