tqma6s_wru4_mmc: manage board_spi_cs_gpio correctly

Define the function board_spi_cs_gpio only when needed,
only called in drivers/spi/mxc_spi.c.
That avoid compilation issue for tqma6s_wru4_mmc_defconfig
when CONFIG_SF_DEFAULT_BUS and CONFIG_SF_DEFAULT_CS are not
defined (CMD_SF not defined) after migration in KConfig.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>