RISC-V: convert new selectors of RISCV_ALTERNATIVE to dependencies

RISC-V: convert new selectors of RISCV_ALTERNATIVE to dependencies

for-next contains two additional extensions that select
RISCV_ALTERNATIVE. RISCV_ALTERNATIVE no longer needs to be selected by
individual config options as it is now selected for !XIP_KERNEL builds
by the top level RISCV option.
These extensions rely on the alternative framework, so convert the
"select"s to "depends on"s instead.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20230324121240.3594777-1-conor.dooley@microchip.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>