boards: stm32n6: Fix flash configuration for twister

On STM32N6 based series, there is no flash available and tests
are performed using load and run in RAM.
From twister point of view, we should consider a flash is available
with the same size as RAM.
(We may tune these values later as the RAM will actually be shared
between code and variables).

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>