ARM: dts: nspire: Fix uart node to conform with DT binding

This node does not follow the DT binding schema, correct this.
The arm,pl011 binding requires the first clock to be named "uartclk".
Should result in no functional change.

Signed-off-by: Andrew Davis <afd@ti.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>