default or selected kernelversion does not have config value CONFIG_ARM_MHU_V2.
Result is shown for kernelversion 5.11-rc4

ARM MHUv2 Mailbox

modulename: arm_mhuv2.ko
configname: CONFIG_ARM_MHU_V2

Linux Kernel Configuration
└─> Device Drivers
└─> Mailbox Hardware Support
└─> ARM MHUv2 Mailbox

Say Y here if you want to build the ARM MHUv2 controller driver,
which provides unidirectional mailboxes between processing elements.

source code: