dt-bindings: timer: tpm: fix typo of clock name

The clock name should be ipg instead of igp.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

Documentation/devicetree/bindings/timer/nxp,tpm-timer.txt

@@ -15,7 +15,7 @@ Required properties:
 - interrupts :	Should be the clock event device interrupt.
 - clocks :	The clocks provided by the SoC to drive the timer, must contain
 		an entry for each entry in clock-names.
-- clock-names : Must include the following entries: "igp" and "per".
+- clock-names : Must include the following entries: "ipg" and "per".
 
 Example:
 tpm5: tpm@40260000 {