ARM: dts: dra7-evm-common: Fix duplicate mailbox nodes

The mailbox nodes defined in various dts files have been moved to
common dra7-ipu-dsp-common.dtsi and dra74-ipu-dsp-common.dtsi files
in commit a11a2f73 ("ARM: dts: dra7-ipu-dsp-common: Move mailboxes
into common files"), but the nodes were erroneously left out in the
dra7-evm-common.dtsi file. Fix this by removing these duplicate nodes.

Fixes: a11a2f73 ("ARM: dts: dra7-ipu-dsp-common: Move mailboxes into common files")
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>