gpio: at91: Fix pullup/pulldown configuration on PIO3

On systems with PIO3 (SAMA5D3/D4/..), the pullup and pulldown configuration
is mutualy exclusive. This patch assures that the opposite pull resistor gets
disabled before the requested pull resistor is enabled. This changes behavior
of at91_set_pio_pulldown() such that the pullup is only disabled if pulldown
is to be enabled. This changes behavior of at91_set_pio_pullup() such that
the pulldown is only disabled if pullup is to be enabled.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Andreas Bießmann <andreas.devel@googlemail.com>
Cc: Josh Wu <josh.wu@atmel.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Andreas Bießmann <andreas@biessmann.org>