serial: 8250_pnp: Support configurable reg shift property

The 16550a serial port based on the ACPI table requires obtaining the
reg-shift attribute. In the ACPI scenario, If the reg-shift property
is not configured like in DTS, the 16550a serial driver cannot read or
write controller registers properly during initialization.

Signed-off-by: Guanbing Huang <albanhuang@tencent.com>
Suggested-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Bing Fan <tombinfan@tencent.com>
Tested-by: Linheng Du <dylanlhdu@tencent.com>
Link: https://lore.kernel.org/r/4726ecea8f7bfbfe42501b4f6ad9fe5b38994574.1713234515.git.albanhuang@tencent.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>