[ARM] pxa/pcm990: start external GPIOs immediately after built-in ones

As a result of an off-by-1 error pcm990 leaves one unused GPIO number between
built-in GPIOs and the pca9536 extender. Fix it.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Eric Miao <eric.miao@marvell.com>