feat(sgi): remove override for `ARM_BL31_IN_DRAM` build-option
RD-N2* variants of Neoverse reference design platforms could be
configured to boot from SRAM or DRAM. Having ARM_BL31_IN_DRAM set to 1
within the common makefile would deter these platforms from having this
flexibility. Remove the default override configuration for
`ARM_BL31_IN_DRAM`.