feat(qemu): make coherent memory section optional

Since CPUs such as cortex-a76 are hardware-assisted coherent, coherent
memory section is not required for them and should be an optional
section.

Signed-off-by: Chen Baozi <chenbaozi@phytium.com.cn>
Change-Id: I03c8e9148ca1780b8af92024359698f4452f7129