lantiq: add the missing function pointer ltq_pci_plat_dev_init

lantiq: add the missing function pointer ltq_pci_plat_dev_init

Partial revert upstream commit 075dd2eef025 ("MIPS: lantiq: Remove
unused function pointer variables") to fix the build error:

mips-openwrt-linux-musl-ld: arch/mips/lantiq/xway/ath5k_eep.o: in function `of_ath5k_eeprom_probe':
arch/mips/lantiq/xway/ath5k_eep.c:(.init.text+0x31c): undefined reference to `ltq_pci_plat_dev_init'
mips-openwrt-linux-musl-ld: arch/mips/lantiq/xway/ath5k_eep.c:(.init.text+0x338): undefined reference to `ltq_pci_plat_dev_init'

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
lantiq: fix vmmc driver build warnings