ethernet: stm32h7: Move DMA buffers from sram3 to sram2

PR #30403 implemented nocache regions for ethernet DMA buffers in sram3 to
fix issue #29915. Unfortunately, some STM32H7 variants do not have any
sram3 so they still suffer from #29915.

All H7 variants have sram2 though, so use that for targets without sram3.

Signed-off-by: Björn Stenberg <bjorn@haxx.se>