ARM: davinci: da8xx: Create DSP device only when assigned memory

The DSP device on Davinci platforms does not have an MMU and requires
specific DDR memory to boot. This memory is reserved using the rproc_mem
kernel boot parameter and is assigned to the device on non-DT boots.
The remoteproc core uses the DMA API and so will fall back to assigning
random memory if this memory is not assigned to the device, but the DSP
remote processor boot will not be successful in such cases. So, check
that memory has been reserved and assigned to the device specifically
before even creating the DSP device.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>