ARM: tegra: emc: convert tegra2_emc to a platform driver

This is the first step in making it device-tree aware and get rid of the
in-kernel EMC tables (of which there are none in mainline, thankfully).

Changes since v3:

* moved to devm_request_and_ioremap() in probe()

Changes since v2:

* D'oh -- missed a couple of variables that were added, never used and then
  later removed in a later patch.

Changes since v1:

* Fixed messed up indentation
* Removed code that should be gone (was added here and removed later in series)

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Stephen Warren <swarren@nvidia.com>