ti_config_fragments: v8_rpmsg: Enable OMAP Mailbox support

Add support to build the OMAP Mailbox driver, required for remote
processor messaging with various R5F and DSP remoteprocs on the TI
K3 family of SoCs. The Mailbox driver is chosen to be built-in by
default.

Signed-off-by: Suman Anna <s-anna@ti.com>