bsp/riscv: Work area size based on /memory node in fdt

Uses the first entry in the /memory node to determine the end of the
work area. Falls back on linker symbol if unable to parse the node.

spec/build/bsps/riscv/riscv/obj.yml

@@ -28,6 +28,7 @@ source:
 - bsps/riscv/riscv/irq/irq.c
 - bsps/riscv/riscv/start/bsp_fatal_halt.c
 - bsps/riscv/riscv/start/bspstart.c
+- bsps/riscv/shared/start/bspgetworkarea.c
 - bsps/shared/cache/nocache.c
 - bsps/shared/dev/btimer/btimer-cpucounter.c
 - bsps/shared/dev/getentropy/getentropy-cpucounter.c