serial: Rename earlycon semihost driver

Now that earlycon semihost driver works on RISC-V too, let's use a
much more generic name for the driver.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Tested-by: Sergey Matyukevich <sergey.matyukevich@syntacore.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Link: https://lore.kernel.org/r/20221209150437.795918-4-bmeng@tinylab.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>