rtio: add helper function `rtio_read_transaction()`

Add a helper function that constructs a rtio SQE chain with the purpose
to perform a bus read operation on a list of registers.

Usage:

  struct rtio_regs regs;
  struct rtio_reg_list regs_list[] = {{regs_addr1, mem_addr_1, mem_len_1},
                                      {regs_addr2, mem_addr_2, mem_len_2},
                                      ...
                                     };
  regs.rtio_regs_list = regs_list;
  regs.rtio_regs_num = ARRAY_SIZE(regs_list);

  rtio_read_regs_async(rtio,
                       iodev,
                       RTIO_BUS_SPI,
                       &regs,
                       sqe,
                       dev,
                       op_cb);

Signed-off-by: Armando Visconti <armando.visconti@st.com>