8250.c: support specifying DW APB UARTs in device platform_data

Allow the private_data field to be specified in platform_data for the
standard 8250/16550 UART.  This field is used by DW APB type UARTs and
without this patch it's only possible to set this field when registering
the port by hand.  If private_data is not set then the driver will
potentially oops with a NULL pointer dereference.

Signed-off-by: Will Newton <will.newton@gmail.com>
Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>