While booting, Lenovo 14ARB7 reports 'lenovo-ymc: Unknown key 0 pressed'
warning. This is caused by lenovo_ymc_probe() calling lenovo_ymc_notify()
at probe time to get the initial tablet-mode-switch state and the key-code
lenovo_ymc_notify() reads from the firmware is not initialized at probe
time yet on the Lenovo 14ARB7.

The hardware/firmware does an ACPI notify on the WMI device itself when
it initializes the tablet-mode-switch state later on.

Add 0x0 YMC state to the sparse keymap to silence the warning.
Signed-off-by: Gergo Koteles <soyer@irl.hu>
Link: https://lore.kernel.org/r/08ab73bb74c4ad448409f2ce707b1148874a05ce.1724340562.git.soyer@irl.hu
[hdegoede@redhat.com: Reword commit message]
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>