ARM: shmobile: ape6evm dts: Enable pull-up for GPIO switches

The switches on r8a73a4/ape6evm do not have pull-up registers.  The
schematics say: "Need to use APE6 internal PullUp", hence enable pull-up
using pinctrl.

Without this, the switches don't really work, as the GPIO inputs are
more likely to pick up ghost signals through capacitive coupling than
actual keypresses.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>