venice: lpddr4_timing_imx8mm: add 4gb single die support

venice: lpddr4_timing_imx8mm: add 4gb single die support

Add dram support for the MT53E1G32D2FW-046 RevC part which is a single die
32Gbit density part vs RevA/B which were dual-die parts:
 - use a previously unused EEPROM byte to denote a variant of the
   base config to be patched
 - add a dram description string
 - return the board struct from eeprom_init and pass it to the
   spl_dram_init function so that it has access to the EEPROM
 - move ddr_init into the spl_dram_init so that it can be patched
   in the per-soc init function

Signed-off-by: Tim Harvey <tharvey@gateworks.com>