dts: Move CONFIG_SRAM_* & CONFIG_FLASH_* back to Kconfig

dts will now generate DT_SRAM_BASE_ADDRESS, DT_SRAM_SIZE,
DT_FLASH_BASE_ADDRESS, and DT_FLASH_SIZE defines.  Kconfig can utilize
these defines to set defaults for the CONFIG_ variants.

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