net: lwm2m: plain-text: process only reads for a specific resource
net: lwm2m: plain-text: process only reads for a specific resource

The plain-text format only supports READ op for a specific resource.
In all other cases return NOT_ALLOWED.

Signed-off-by: Michael Scott <a17fed27eaa842282862ff7c1b9c8395a26ac320@foundries.io>

punitvara/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,kraj/zephyr,nashif/zephyr,explora26/zephyr,finikorg/zephyr,explora26/zephyr,Vudentz/zephyr,kraj/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,punitvara/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,ldts/zephyr,galak/zephyr,explora26/zephyr,ldts/zephyr,kraj/zephyr,nashif/zephyr,punitvara/zephyr,Vudentz/zephyr,kraj/zephyr,kraj/zephyr,Vudentz/zephyr,ldts/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,ldts/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,punitvara/zephyr,explora26/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr