serial: 8250_dw: split Synopsys DesignWare 8250 common functions

[ Upstream commit 136e0ab99b22378e3ff7d54f799a3a329316e869 ]

We would like to use same functions in the couple of drivers for
Synopsys DesignWare 8250 UART. Split them from 8250_dw into new brand
library module which users will select explicitly.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://lore.kernel.org/r/20190806094322.64987-3-andriy.shevchenko@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Stable-dep-of: 748c5ea8b879 ("serial: 8250_dw: Preserve original value of DLF register")
Signed-off-by: Sasha Levin <sashal@kernel.org>