dm: gpio: uclass: Add flag to control sequence numbering

Like SPI and I2C few GPIO controllers also have
multiple chip instances. This patch adds the
flag 'DM_UC_FLAG_SEQ_ALIAS' in gpio_uclass driver
to control device sequence numbering. By defalut
the dev->r_seq for gpio_uclass will alwalys
returns -1, which leads the gpio driver probe
failure when using the driver with device trees.

Signed-off-by: Bhuvanchandra DV <bhuvanchandra.dv@toradex.com>