ARM: dts: AM4372: Correct mailbox node data

The mailbox DT node for AM4372 is enabled and is corrected to
remove some properties that have crept in by mistake.

Fixes: 9e3269b8

 (ARM: dts: AM4372: Add L2, EDMA, mailbox, MMC and SHAM nodes)
Cc: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>