dts: arm: Introduce st/mem.h for FLASH & SRAM sizes

As there can be a large number of FLASH & SRAM sizes for the same SoC
having a dts/dtsi for each one would be extremely painful.  Lets just
use some #defines to set that FLASH & SRAM sizes based on the SoC that
is being built.

Change-Id: I06388ada4e49ed3d576da31150288512bb6b4485
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>