Enable i.MX thermal DM driver to mx6sabre_common.h file. Since the
thermal is used in init_sequence_f, so define the CONFIG_SYS_MALLOC_F_LEN
to support DM driver using in pre relocation phase.
Additional, thermal driver depends on ocotp, make sure to enable
CONFIG_MXC_OCOTP when CONFIG_IMX6_THERMAL is selected.