ARM: dts: OMAP5: Add mailbox dt node to fix boot warning

Add the mailbox device DT node for OMAP5 SoC. The OMAP5 mailbox
IP is identical to that used in OMAP4.

The OMAP5 hwmod data no longer publishes the module address space,
so this patch fixes the WARN_ON backtrace associated with the
following trace during the kernel boot:
"omap_hwmod: mailbox: doesn't have mpu register target base".

Otherwise we get a warning like this:

WARNING: CPU: 0 PID: 1 at arch/arm/mach-omap2/omap_hwmod.c:2538 _init+0x1c0/0x3dc()
omap_hwmod: mailbox: doesn't have mpu register target base
Modules linked in:
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.15.0-rc2-00001-gb5e85a0 #45
[<c0015724>] (unwind_backtrace) from [<c00120f4>] (show_stack+0x10/0x14)
[<c00120f4>] (show_stack) from [<c05a1ccc>] (dump_stack+0x78/0x94)
[<c05a1ccc>] (dump_stack) from [<c0042a74>] (warn_slowpath_common+0x6c/0x8c)
[<c0042a74>] (warn_slowpath_common) from [<c0042b28>] (warn_slowpath_fmt+0x30/0x40)
[<c0042b28>] (warn_slowpath_fmt) from [<c0803b40>] (_init+0x1c0/0x3dc)
[<c0803b40>] (_init) from [<c0029c8c>] (omap_hwmod_for_each+0x34/0x5c)
[<c0029c8c>] (omap_hwmod_for_each) from [<c08042b0>] (__omap_hwmod_setup_all+0x24/0x40)
[<c08042b0>] (__omap_hwmod_setup_all) from [<c00088b8>] (do_one_initcall+0x34/0x160)
[<c00088b8>] (do_one_initcall) from [<c07f7bf4>] (kernel_init_freeable+0xfc/0x1c8)
[<c07f7bf4>] (kernel_init_freeable) from [<c059c4f4>] (kernel_init+0x8/0xe4)
[<c059c4f4>] (kernel_init) from [<c000eaa8>] (ret_from_fork+0x14/0x2c)

Signed-off-by: Suman Anna <s-anna@ti.com>
[tony@atomide.com: updated description to for the warning]
Signed-off-by: Tony Lindgren <tony@atomide.com>