platform/x86: Add new get_serdev_controller() helper

In some cases UART attached devices which require an in kernel driver,
e.g. UART attached Bluetooth HCIs are described in the ACPI tables
by an ACPI device with a broken or missing UartSerialBusV2() resource.

This causes the kernel to create a /dev/ttyS# char-device for the UART
instead of creating an in kernel serdev-controller + serdev-device pair
for the in kernel driver.

The quirk handling in acpi_quirk_skip_serdev_enumeration() makes the kernel
create a serdev-controller device for these UARTs instead of a /dev/ttyS#.

Instantiating the actual serdev-device to bind to is up to pdx86 code,
so far this was handled by the x86-android-tablets code. But since
commit b286f4e87e32 ("serial: core: Move tty and serdev to be children of
serial core port device") the serdev-controller device has moved in the
device hierarchy from (e.g.) /sys/devices/pci0000:00/8086228A:00/serial0 to
/sys/devices/pci0000:00/8086228A:00/8086228A:00:0/8086228A:00:0.0/serial0 .

This makes this a bit trickier to do and another driver is in the works
which will also need this functionality.

Add a new helper to get the serdev-controller device, so that the new
code for this can be shared.

Fixes: b286f4e87e32 ("serial: core: Move tty and serdev to be children of serial core port device")
Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20240216201721.239791-3-hdegoede@redhat.com