samples: bluetooth: hci_rpmsg: use new-style DT for shared mem macro
samples: bluetooth: hci_rpmsg: use new-style DT for shared mem macro

Adapt to use new-style Device Tree macros for the
shared memory defines.

Signed-off-by: Ioannis Glaropoulos <5921cc8bab7e1d4329f52fd8f6268f9692e3de80@nordicsemi.no>

Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr