As not all drivers support interrupts we don't have a means to know if
we can allow UART_INTERRUPT_DRIVEN to be enabled and thus various
Kconfig options that select UART_INTERRUPT_DRIVEN. Some drivers have a
interrupts, however not all do. So introduce a Kconfig option to let us
know what the driver actually supports.
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>