samples: code_relocation_nocopy: update macro to get flash size

The flash size is the second part (size) of the first reg value,

not the first part (address) of a nonexistent second reg value.

DT_REG_SIZE get a node's (only) register block size instead
DT_REG_ADDR_BY_IDX .

Signed-off-by: Fabrice DJIATSA <fabrice.djiatsa-ext@st.com>