arm: st: dts: remove last !HAS_DTS from stm32 SoCs

Since all STM32 SoCs are using device tree we can remove this last bit
of !HAS_DTS for getting flash/sram sizes from Kconfig.

Change-Id: I9e706b7aba7c0edcf9fca3ddc0ddc7d820980b47
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>