soc: nios2: Cleanup linker scripts to use new DTS macros

Move defines for _RAM_ADDR, _RAM_SIZE, _ROM_ADDR, and _ROM_ADDR into
the linker.ld and thus remove dts_fixup.h.  We rework to use
DT_REG_ADDR and DT_REG_SIZE on DT_CHOSEN(zephyr_sram) and
DT_CHOSEN(zephyr_flash).

Also fixup use of _RAM_ADDR/_RAM_SIZE in newlib/libc-hooks.c.

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