serial: Fix SIFIVE debug serial dependency

The commit 4cc24aea ("serial: Add missing Kconfig dependencies for
debug consoles") has added incorrect dependency for SIFIVE debug uart which
should depend on SIFIVE driver instead of PL01x.

Fixes: 4cc24aea ("serial: Add missing Kconfig dependencies for debug consoles")
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Leo Liang <ycliang@andestech.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>