soc: riscv: openisa_rv32m1: Conver to new devicetree macros

Convert DT_CODE_PARTITION_{OFFSET,SIZE} to use new
DT_REG_ADDR/DT_REG_SIZE macros instead based on
DT_CHOSEN(zephyr_code_partition).

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