[ARM] Fix AMBA serial drivers for non-first serial ports

Using console=ttyAM1 or console=ttyAMA1 resulted in an oops during
boot due to trying to drive the console before that port had been
registered.  Fix this by checking whether the port is present before
allowing console setup to proceed.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>