runqemu: avoid setting "mem=" when using a DTB

runqemu: avoid setting "mem=" when using a DTB

Some emulated machines, such as the Raspberry Pi models, fail to boot
if a fixed "mem=" value is passed on the kernel command line. When a
DTB is used (QB_DTB is set), the memory size is already defined there,
so specifying it again is unnecessary and can cause issues.

Skip adding "mem=" to the kernel command line when QB_DTB is present.

Signed-off-by: Jon Mason <jon.mason@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>