soc: riscv32: openisa_rv32m1: Link .srodata section

Apply a similar fix to commit 0289a410
to the openisa_rv32m1 linker script for handling .srodata sections.

Otherwise we get oprhan read-only data section.  Fix this by adding
the .srodata section to the RISC-V linker script.

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