serial: 8250: SERIAL_8250_EM should depend on ARCH_RENESAS

The Emma Mobile integrated serial port hardware is only present on Emma
Mobile SoCs.  Hence add a dependency on ARCH_RENESAS, to prevent asking
the user about this driver when configuring a kernel without Renesas
ARM32 SoC support.

Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/7b5a4bbf2f47b2c4c127817e8b1524a650795d97.1631710085.git.geert+renesas@glider.be


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