pcnet: access descriptor rings & init block uncached

The prior accesses to the descriptor rings & init block via cached
memory had a few issues:

  - The memory needs cache flushes or invalidation at the appropriate
    times, but was not necessarily aligned on cache line boundaries.
    This could lead to data being incorrectly lost or written back to
    RAM at the wrong time.

  - There are points where ordering of writes to the memory is
    important, but because it's cached memory the pcnet controller
    would see cache lines written back ordered by address. This could
    occasionally lead to hardware seeing descriptors in an incorrect
    state.

  - Flushing the cache constantly is inefficient.

So, to avoid all of those issues simply access the descriptors & init
block via uncached memory. The MIPS-specific UNCACHED_SDRAM macro is
used to do this (retrieving an address in kseg1) as I could see no
existing generic solution. Since the MIPS Malta board is the only user
of the pcnet driver, hopefully this doesn't matter.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>