ARM: dts: imx27-eukrea-cpuimx27: Put the clock node directly into root

Put the UART clock node directly under root with a unique node name and
drop unit-address in order to fix the following build warning with W=1:

arch/arm/boot/dts/imx27-eukrea-mbimxsd27-baseboard.dtb: Warning (simple_bus_reg): Node /clocks/osc26m missing or empty reg/ranges property

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>