drivers: mipi_dbi: update macro to get address

DT_REG_ADDR now generates an unsigned string terminated with U
which doesn't match the way the macros is used in a CONCAT
to build a FMC_BANK1_(1/2/3) define that is defined in hal.
`DT_REG_ADDR_RAW` should now be used to get the RAW FMC bank index

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