[vpp]: add platform vpp (#20841)

Why I did it
Add platform vpp to sonic-buildimage to enable building sonic-vpp as other platforms.

How I did it
Add vpp submodule from https://github.com/sonic-net/sonic-platform-vpp

How to verify it
git clone --recurse-submodules https://github.com/sonic-net/sonic-buildimage.git
make init
make configure PLATFORM=vpp
make SONIC_BUILD_JOBS=4 target/sonic-vpp.img.gz

Signed-off-by: Yue Gao <yuega2@cisco.com>

Author: yue-fred-gao

.gitmodules

@@ -130,3 +130,6 @@
 [submodule "platform/marvell-teralynx/sonic-platform-marvell-teralynx"]
 	path = platform/marvell-teralynx/sonic-platform-marvell-teralynx
 	url = https://github.com/Marvell-switching/sonic-platform-marvell.git
+[submodule "platform/vpp"]
+	path = platform/vpp
+	url = https://github.com/sonic-net/sonic-platform-vpp.git