assists:baremetallinker: Avoid setting axi_emc as the default memory in linker sections

[onkarharsh]

axi_emc is a softIP used to interface with external memory devices. The block provides memory controller functionality primarily for parallel NOR Flash and linear flash devices. ELFs can't be directly downloaded over axi_emc using "dow" and a separate flash utility is needed. Therefore, avoid setting axi_emc as the default memory in all the linker sections. This is a makeshift solution until all the data and code sections are properly divided among the available memory IPs.

[onkarharsh]

@kedareswararao @sivadur : your other PR may need a rebase before v2.