soc: riscv: esp32c3: apply CONFIG_RISCV_GP option

esp32c3 has already supported RISC-V GP, just apply new kconfig option
to it. Forcely select CONFIG_RISCV_GP in esp32c3 at first because it
seems to be necessary in the esp32c3 SoC.

Signed-off-by: Jim Shu <cwshu@andestech.com>