Change tensor arena size to be a power of two

The size of the memory used for the message handler's tensor arena will
always be rounded up to the next power of two when the Linux kernel
allocates it. Therefore, to align the tensor arena size with how the
memory is allocated and be able to use the full size allocated, the
tensor arena size must now be a power of two.

Change-Id: Ib67e77d1ffeb233ad12762a56ddbc20d26c71052
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>