boards: stm32n6570_dk/nucleo_n657x0_q: Set NOR XSPI controller clk to HCLK5

Fix copy/paste issue on XSPI_SEL macro and use HCLK5 as kernel clk.
This has no functional impact on NOR as HCLK5 is the default clk which
was used due to the copy/paste error.
Remove now useless ic3 nodes.

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