dt-bindings: rtc: nxp,lpc1788-rtc: add compatible string nxp,lpc1850-rtc

dt-bindings: rtc: nxp,lpc1788-rtc: add compatible string nxp,lpc1850-rtc

Add compatible string nxp,lpc1850-rtc and fallback to nxp,lpc1788-rtc.

Fix below CHECK_DTB warning:
  arch/arm/boot/dts/nxp/lpc/lpc4337-ciaa.dtb: rtc@40046000 (nxp,lpc1850-rtc): compatible: ['nxp,lpc1850-rtc', 'nxp,lpc1788-rtc'] is too long

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250602142842.942700-1-Frank.Li@nxp.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>