serial: SERIAL_SUNPLUS should depend on ARCH_SUNPLUS

Sunplus serial ports are only present on Sunplus SoCs.  Hence add a
dependency on ARCH_SUNPLUS, to prevent asking the user about this driver
when configuring a kernel without Sunplus SoC support.

Fixes: 9e8d5470 ("serial: sunplus-uart: Add Sunplus SoC UART Driver")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/59f46272ab5b16853acac4d585c3333cfd394223.1647352195.git.geert+renesas@glider.be


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>