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>