qcom: cmd-db: enforce CONFIG_OF_RESERVED_MEM dependency

Without CONFIG_OF_RESERVED_MEM, gcc sees that the global cmd_db_header
variable is never initialized, and through code optimization concludes
that a lot of other code cannot possibly work after that:

drivers/soc/qcom/cmd-db.c: In function 'cmd_db_read_addr':
drivers/soc/qcom/cmd-db.c:197:21: error: 'ent.addr' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  return ret < 0 ? 0 : le32_to_cpu(ent.addr);
drivers/soc/qcom/cmd-db.c: In function 'cmd_db_read_aux_data':
drivers/soc/qcom/cmd-db.c:224:10: error: 'ent.len' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  ent_len = le16_to_cpu(ent.len);
drivers/soc/qcom/cmd-db.c:115:6: error: 'rsc_hdr.data_offset' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  u16 offset = le16_to_cpu(hdr->data_offset);
      ^~~~~~
drivers/soc/qcom/cmd-db.c:116:6: error: 'ent.offset' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  u16 loffset = le16_to_cpu(ent->offset);
      ^~~~~~~
drivers/soc/qcom/cmd-db.c: In function 'cmd_db_read_aux_data_len':
drivers/soc/qcom/cmd-db.c:250:38: error: 'ent.len' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  return ret < 0 ? 0 : le16_to_cpu(ent.len);
                                      ^
drivers/soc/qcom/cmd-db.c: In function 'cmd_db_read_slave_id':
drivers/soc/qcom/cmd-db.c:272:7: error: 'ent.addr' may be used uninitialized in this function [-Werror=maybe-uninitialized]

Using a hard CONFIG_OF_RESERVED_MEM dependency avoids this warning,
and we can remove the CONFIG_OF dependency.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>