linker: common-rom-logging: Use DEVNULL_REGION only if it exists

Log string removal is by default enabled for RISCV and Cortex-M
and support is added to the default linker template. However, if
SoC is not using default linker template then DEVNULL_REGION is
not defined. In that case string removal cannot be used.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>