ARM: dts: OMAP2420: Drop interrupt-names from mailbox node

The interrupt-names property is neither defined nor used in either
of the OMAP Mailbox binding or the driver. So, drop them. This is
in preparation for converting the OMAP Mailbox binding to YAML
format.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>