dm: spi: Read default speed and mode values from DT

This patch update the behavior introduced by
commit 96907c0f ("dm: spi: Read default speed and mode values from DT")

In case of DT boot, don't read default speed and mode for SPI from
CONFIG_* but instead read from DT node. This will make sure that boards
with multiple SPI/QSPI controllers can be probed at different
bus frequencies and SPI modes.

Remove also use in boards of the value speed=0 (no more supported)
for ENV in SPI by using CONFIG_ENV_SPI_MAX_HZ=0.

DT values will be always used when available (full DM support of
SPI slave with available DT node) even if speed and mode are requested;
for example in splash screen support (in splash_sf_read_raw)
or in SPL boot (in spl_spi_load_image).
The caller of spi_get_bus_and_cs() no more need to force speed=0.

But the current behavior don't change if the SPI slave is not
present (device with generic driver is created automatically)
or if platdata is used (CONFIG_OF_PLATDATA).

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>