Fix mbed_hal-watchdog failing with OOM
Fix mbed_hal-watchdog failing with OOM

This is to fix OOM error on targets with just 16KiB RAM.

andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,kjbracey-arm/mbed,mbedmicro/mbed,mbedmicro/mbed