mb/google/volteer/variants/eldrid: add memory.c for ddr4 support

mb/google/volteer/variants/eldrid: add memory.c for ddr4 support

Add new memory.c to support DDR4 memory types.

Use the new meminit_ddr() and variant_memory_sku() for eldrid variant
code on memory.c

The initial settings override the baseboard from volteer and fine tune
gpio.c and overridetree.cb on eldrid's configuration.

BUG=b:161772961
TEST='emerge-volteer coreboot chromeos-bootimage' and verify that Eldrid
can boots. NOTE that tests the ddr4 side of the implementation.

Change-Id: I2c7b30093a8d85bac5aba5b83768af5eb36c4f70
Signed-off-by: Nick Chen <nick_xr_chen@wistron.corp-partner.google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44632
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Caveh Jalali <caveh@chromium.org>