soc: arc: Use new dts macros in linker scripts and arc_mpu_regions

Convert linker scripts and arc_mpu_regions.c setup to use new
devicetree.h macros to extract the base address and size of the various
memory regions (DDR, SRAM, FLASH, DCCM, ICCM).  We also remove the
scaling up and down since DT_REG_SIZE() returns the value in bytes.

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