driver: gpio: mcp23xxx: increase reset pin pulse duration

The reset pulse is currently fixed at 1 µs, the minimum required for
the chip. However, a long reset pin trace can increases rise time, making
1 µs potentially insufficient for reliable detection of a reset signal.
Increase the pulse duration to 2 µs

Signed-off-by: Igor Knippenberg <igor.knippenberg@gmail.com>