riscv: andesv5: Prefer using the generic RISC-V timer driver in S-mode

The Andes PLMT driver directly accesses the mtime MMIO region,
indicating its intended use in the M-mode boot stage. However,
since U-Boot proper (S-mode) also uses the PLMT driver, we need
to specifically mark the region as readable through PMPCFGx (or
S/U-mode read-only shared data region for Smepmp) in OpenSBI.

Granting permission for this case doesn't make sense. Instead,
we should use the generic RISC-V timer driver to read the mtime
through the TIME CSR. Therefore, we add the SPL_ANDES_PLMT_TIMER
config, which ensures that the PLMT driver is linked exclusively
against M-mode U-Boot or U-Boot SPL binaries.

Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Samuel Holland <samuel@sholland.org>