boards: nucleo_wb55rg: Flash partition not compatible w/ BLE f/w 1.13

Update board flash partition to be compatible with v1.13.0 f/w.
This configuration is compatible with stm32wb5x_BLE_Stack_full_fw
which should be installed at 0x080C7000.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>