ipc: icbmsg: Reduce block alignment to 32-bits

The ICBMsg backend divides its memory into
blocks. Each block is aligned to data cache
alignment. Is it not required, since adjacent
blocks has the same data flow direction (either
read-only or write-only). This commit changes
it to 32-bits making wasted memory significantly
reduced.

Signed-off-by: Dominik Kilian <Dominik.Kilian@nordicsemi.no>