Newer eSDHC controllers support ADMA2 descriptor tables which support
64bit DMA addresses. One notable user of addresses in the upper memory
segment is the EFI loader.

If support is enabled, but the controller doesn't support ADMA2, we
will fall back to SDMA (and thus 32 bit DMA addresses only).

Signed-off-by: Michael Walle <michael@walle.cc>