riscv: linker: replace DT_FLASH_{BASE_ADDRESS,SIZE} with new macros

The riscv linker scripts utilize DT_FLASH_BASE_ADDRESS and
DT_FLASH_SIZE, as we want to phase out the old generator we need to
replace these defines with macros from devicetree.h.

We support two flash configurations at this point, either a QSPI flash
like on the hifive board or a SoC flash like on the rv32m1_vega.  We
update the linker scripts to check the compat of the zephyr,flash node
and based on if its 'jedec,spi-nor' or 'soc-nv-flash' we determine how
to extract the "flash" base address and size.

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