ipc: static_vrings: Support DT-defined buffer size

Recently OpenAMP introduced the possibility to set the sizes for TX and
RX buffers per created instance. Expose this also to Zephyr users by
using a DT property "zephyr,buffer-size".

For the sake of simplicity use the same DT property to set the buffer
size for both TX and RX buffers.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>