default or selected kernelversion does not have config value CONFIG_OMAP_MBOX.
Result is shown for kernelversion 6.5.1

CONFIG_OMAP_MBOX (not configurable)

modulename: omap-mailbox.ko

configname: CONFIG_OMAP_MBOX

Linux Kernel Configuration
└─>Device Drivers
└─>Mailbox Hardware Support
└─>OMAP_MBOX
In linux kernel since version 3.11 (release Date: 2013-09-02)  
This option is selected by any OMAP architecture specific mailbox
driver such as CONFIG_OMAP1_MBOX or CONFIG_OMAP2PLUS_MBOX. This
enables the common OMAP mailbox framework code.

source code: