runqemu: remove setting of mem on kernel command line for certain systems

runqemu: remove setting of mem on kernel command line for certain systems

Some emulated hardware will not boot if mem is set on the kernel command
line (all of the Raspberry Pi machines seemed to fail with this set,
possibly many others).  Also, it is not necessary if the device tree
file is present, as that _should_ have the memory size specified in it.
Add a check for QB_DTB and don't set mem in the kernel command line if
present.

Signed-off-by: Jon Mason <jon.mason@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>