ARM: dts: silk: Configure pull-up for SOFT_SW GPIO keys

The GPIO pins connected to the 4 Software Switches ("SOFT_SW", SW12) do
not have external pull-up resistors, but rely on internal pull-ups being
enabled.  Fortunately this is satisfied by the initial state of these
pins.

Make this explicit by enabling bias-pull-up, to remove the dependency on
initial state and/or boot loader configuration.

While at it, rename the surrounding device node from "gpio-keys" to
"keyboard", to comply with generic node name recommendations.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/45f38a5333feba9bea80efeb5a41a6c3f60deda2.1619785905.git.geert+renesas@glider.be