[Arista] Fix Clearwater2 phy initialization when no configuration is provided (#8271)

Why I did it
Fix an issue on the Clearwater2 linecard.
When the linecard is started with a fresh image without configuration, phys would not be initialized.

How I did it
Added default_sku for Clearwater2 which prevents config-setup from failing to create a default config_db.json.
Added some extra logic in the phy-credo-init script to run the phy_config.sh of the hwsku pointed by default_sku if the DEVICE_METADATA.localhost.hwsku information is not populated in CONFIG_DB.

How to verify it
Booting an image with this change and without configuration will lead to the phys being initialized using the phy_config.sh from default_sku.

device/arista/x86_64-arista_7800r3_48cq2_lc/default_sku

@@ -0,0 +1 @@
+Arista-7800R3-48CQ2-C48 t1

device/arista/x86_64-arista_7800r3_48cqm2_lc/default_sku

@@ -0,0 +1 @@
+Arista-7800R3-48CQM2-C48 t1

rules/phy-credo.mk

@@ -1,3 +1,3 @@
 PHY_CREDO = phy-credo_1.0_amd64.deb
-$(PHY_CREDO)_URL = "https://github.com/aristanetworks/sonic-firmware/raw/08cbc09437e942c1e3cd84a7595ca686193d311b/phy/phy-credo_1.0_amd64.deb"
+$(PHY_CREDO)_URL = "https://github.com/aristanetworks/sonic-firmware/raw/0468f6c23a7219dfee6dd2819013e9f1e940c95d/phy/phy-credo_1.0_amd64.deb"
 SONIC_ONLINE_DEBS += $(PHY_CREDO)