ata: libahci_platform: Parse ports-implemented property in resources getter

The ports-implemented property is mainly used on the OF-based platforms
with no ports mapping initialized by a bootloader/BIOS firmware. Seeing
the same of_property_read_u32()-based pattern has already been implemented
in the generic AHCI LLDD (glue) driver and in the Mediatek, St AHCI
drivers let's move the property read procedure to the generic
ahci_platform_get_resources() method. Thus we'll have the forced ports
mapping feature supported for each OF-based platform which requires that,
and stop re-implementing the same pattern in there a bit simplifying the
code.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>