arm64: zynqmp: Add linux,code for fwuen button

BTN_MISC looks like the most reasonable option for this button.
Button is used by firmware to indicate (after reset, power up) that user
wants to do firmware upgrade via firmware update utility.
For bootloader or OS is this just user button which is worth to have it
mapped.
Also button can be used as a wakeup source and pressing it for more time
can generate more chars that's why also adding wakeup-source and autorepeat
properties.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@amd.com>
Link: https://lore.kernel.org/r/7f6d627473632c3c3036ec9f6aaa36e00f4615e2.1652262769.git.michal.simek@amd.com