serial: 8250: describe CONFIG_SERIAL_8250_RSA

CONFIG_SERIAL_8250_RSA has waited for a long
time to have meaningful help text so let's
finally describe what this option actually does.

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>