RISC-V: Force select RISCV_INTC for CONFIG_RISCV

The RISC-V per-HART local interrupt controller driver is mandatory
for all RISC-V system (with/without MMU) hence we force select it
for CONFIG_RISCV (just like RISCV_TIMER).

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>