warp7: Define a platform_def.h

This patch defines a platform_def.h describing

- FIP layout and location
- eMMC device select
- UART identity select
- System clock frequency
- Operational memory map

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Jun Nie <jun.nie@linaro.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>