riscv: add run mode configuration for SPL

U-Boot SPL can be run in a different privilege mode from U-Boot proper.
Add new configuration entries for SPL to allow the run mode to be
configured independently of U-Boot proper.

Extend all uses of the CONFIG_RISCV_SMODE and CONFIG_RISCV_MMODE
configuration symbols to also cover the SPL equivalents. Ensure that
files compatible with only one privilege mode are not included in builds
targeting an incompatible privilege mode.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>