ARM: imx: Fix mmdc_pmu_write_accesses event definition

Fixed an error in the "write-accesses" event definition.

Signed-off-by: Tiberiu Breana <andrei-tiberiu.breana@nxp.com>
Acked-by: Frank Li <Frank.li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>