net: lwm2m: Allow configuring update period

Add new configuration value CONFIG_LWM2M_UPDATE_PERIOD
that allows calculating update period from last update,
instead of calculating it from the lifetime.

In runtime, server is allowed to change the lifetime of the
registration which causes update perdiod to be effected.

When fixed update period is preferred, UPDATE_PERIOD
config is then used.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>