i compiled a kernel with an initramfs, when i logged into the device i found something strange, the eth0 device is not detected by the kernel but lspci detects a network controller on the board.

root@generic-mips:/# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever

root@generic-mips:/# lspci -m
00:00.0 "Class 0600" "Vendor 184e" "Device 1001" "" ""
00:00.4 "Class 0800" "Vendor 184e" "Device 1003" "" ""
00:01.0 "Class 0604" "Vendor 184e" "Device 1004" "" ""
00:03.0 "Class 0280" "Vendor 184e" "Device 1009" "" ""
00:03.1 "Class 0280" "Vendor 184e" "Device 100a" "" ""
00:04.0 "Class ff00" "Vendor 184e" "Device 100b" "" ""
00:04.1 "Class 0c03" "Vendor 184e" "Device 101d" -p30 "" ""
00:04.2 "Class 0c03" "Vendor 184e" "Device 101d" -p30 "" ""
00:04.3 "Class 0c03" "Vendor 184e" "Device 101d" -p30 "" ""
...

i think i am missing some kind of driver itself

Good move, although it's better as a first step (rather than a sanity check) in developing a kernel for a board IMO.

There's the Ethernet PHY driver, and the entire network protocol stack with all of its optional pieces.
You can try to figure it out, or post the boot log and/or the kernel .config file.

Regards