ARM: dts: stm32: fix dmamux reg property on stm32mp151

Reg property length should cover all DMAMUX_CxCR registers.
DMAMUX_CxCR Address offset: 0x000 + 0x04 * x (x = 0 to 15), so latest
offset is at 0x3c, so length should be 0x40.

Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>