ARM: dts: imx6ul: Enable the PMU node

There is no need to keep the PMU disabled.

Enable it like it is done in the other i.MX dtsi files.

With this change applied we see:

[    1.338866] hw perfevents: enabled with armv7_cortex_a7 PMU driver, 5 counters available

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