ARM: tegra: convert device tree files to use key defines

Use key code macros for all key code refernced for keys.

For tegra20-seaboard.dts and tegra20-harmony.dts:
  The key comment for key (16th row and 1st column) is KEY_KPSLASH but
  code is 0x004e which is the key code for KEY_KPPLUS. As there other
  key exist with KY_KPPLUS, I am assuming key code is wrong and comment
  is fine. With this assumption, I am keeping the key code as KEY_KPSLASH.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>