usb: device_next: introduce UDC_BUF_POOL_*_DEFINE macros

Introduce UDC_BUF_POOL_*_DEFINE macros based on NET_BUF_POOL_*_DEFINE
macros but use our own version of alloc and unref callbacks to get
buffers with specific alignment and granularity. Also, do not use ref
callback because it breaks alignment.
Also introduces helper macros for defining and checking UDC
driver-compliant static buffers.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>