tty: powerpc: remove SERIAL_ICOM dependency on PPC_ISERIES

The PowerPC legacy iSeries platform is being removed so this is no
longer selectable.

Cc: Alan Cox <alan@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-serial@vger.kernel.org
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>