net: extract function net_sntp_set_rtc() from sntp_handler()

net: extract function net_sntp_set_rtc() from sntp_handler()

Extract the code that sets the RTC clock from sntp_handler() in
net/sntp.c and make it a new function net_sntp_set_rtc() in
net/net-common.c. This will allow re-use with NET_LWIP.

According to [1] it is safe to assume that all devices have been
converted to DM_RTC so drop the useless code.

[1] https://lists.denx.de/pipermail/u-boot/2025-June/591376.html

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>