ARM: dts: marzen: Add slide switches

Describe the four General Purpose Switches on the Marzen development
board, so they can be used for user input and/or for wake-up from s2ram.

The GPIO block on R-Car H1 does not support triggering interrupts on
both edges of a changing input signal, hence one cannot use gpio-keys
with gpios properties.  Instead, one of two alternatives needs to be
used:
  1. Use gpio-keys with interrupts instead of gpios properties, at the
     expense of receiving only key presses (release events will be
     auto-generated),
  2. Use gpio-keys-polled with gpios properties, at the expense of
     making these keys unusable as wake-up sources.

As the DTS for the Marzen development board serves mainly as an example,
the approach taken is to use the first alternative for the first two
switches, and the second alternative for the last two switches.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/f834a3c397362f2424fcae6a0c0440356208b182.1679329829.git.geert+renesas@glider.be