ARC: [arcfpga] Early ARC UART to be only activated by cmdline

Earlier CONFIG_SERIAL_ARC_CONSOLE would unconditionally enable the
legacy ARC UART based early console even if platform lacked that UART
(e.g. nSIM OSCI platform).

So a multi-platform image containing the 2 UART drivers won't work.

Fix that by probing the driver only if "earlyprintk=ttyARC0" is present
in cmdline.

Reported-by: Chuck Jordan <cjordan@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>