Failing to insert a custom Kernel module - linux

Question

I'm working on a STM32MP1 target with yocto. In general I can compile a helloworld .ko file and load it into the kernel. But when I want to compile following:
#include <linux/platform_device.h>
#include <linux/module.h>
#include <linux/types.h>
static struct platform_device *pdev;
static int __init fake_eth_add(void)
{
int inst_id = 0;
pdev = platform_device_alloc("fake-eth", inst_id);
platform_device_add(pdev);
pr_info("fake-eth added");
return 0;
}
static void __exit fake_eth_put(void)
{
pr_info("fake-fake removed");
platform_device_put(pdev);
}
module_init(fake_eth_add);
module_exit(fake_eth_put);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("John Madieu <john.madieu#gmail.com>");
I get following errors:
ERROR: modpost: "platform_device_put" [/home/rdm/ethtest/eth-ins.ko] undefined!
ERROR: modpost: "platform_device_add" [/home/rdm/ethtest/eth-ins.ko] undefined!
ERROR: modpost: "platform_device_alloc" [/home/rdm/ethtest/eth-ins.ko] undefined!
I check if the platform_device_alloc Symbol/Function is defined and it does:
root#txmp-1570:/lib/modules/5.7.1/source/kernel# grep -w platform_device_alloc /proc/kallsyms
c0547cfc T platform_device_alloc
Now I have learned that the symbols must be in the Module.symvers File tagged as EXPORT_SYMBOL_GPL. The symbols were not there, so I added them to the Module.symvers file:
0x00000000 platform_device_alloc vmlinux EXPORT_SYMBOL_GPL
0x00000000 platform_device_add vmlinux EXPORT_SYMBOL_GPL
0x00000000 platform_device_put vmlinux EXPORT_SYMBOL_GPL
Now the code compiles! But when I want to load it with insmod, then I get following error:
root#txmp-1570:/home/rdm/ethtest# insmod eth-ins.ko
insmod: ERROR: could not insert module eth-ins.ko: Unknown symbol in module
And the dmseg logs:
[ 2954.658383] eth_ins: Unknown symbol platform_device_put (err -2)
[ 2954.664194] eth_ins: Unknown symbol platform_device_add (err -2)
[ 2954.672737] eth_ins: Unknown symbol platform_device_alloc (err -2)
What am I missing? Why did I have to append the Module.symvers file by myself? Did I miss something regarding setting up the enviromnent for kernel driver development?