riscv: add Kconfig entries for the code model

riscv: add Kconfig entries for the code model

RISC-V has two code models, medium low (medlow) and medium any (medany).
Medlow limits addressable memory to a single 2 GiB range between the
absolute addresses -2 GiB and +2 GiB. Medany limits addressable memory
to any single 2 GiB address range.

By default, medlow is selected for U-Boot on both 32-bit and 64-bit
systems.

The -mcmodel compiler flag is selected according to the Kconfig
configuration.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
[bmeng: adjust to make medlow the default code model for U-Boot]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>