Convert CONFIG_ENV_SPI_* to Kconfig

This converts the following to Kconfig:
  CONFIG_ENV_SPI_BUS
  CONFIG_ENV_SPI_CS
  CONFIG_ENV_SPI_MAX_HZ
  CONFIG_ENV_SPI_MODE

Most of time these value are not needed, CONFIG_SF_DEFAULT
with same value is used, so I introduced CONFIG_USE_ENV_SPI_*
to force the associated value for the environment.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/configs/maxbcm_defconfig b/configs/maxbcm_defconfig
index 5c716cd..1521392 100644
--- a/configs/maxbcm_defconfig
+++ b/configs/maxbcm_defconfig
@@ -30,6 +30,8 @@
 CONFIG_CMD_PING=y
 CONFIG_CMD_TIME=y
 CONFIG_DEFAULT_DEVICE_TREE="armada-xp-maxbcm"
+CONFIG_USE_ENV_SPI_MAX_HZ=y
+CONFIG_ENV_SPI_MAX_HZ=50000000
 CONFIG_SPL_OF_TRANSLATE=y
 # CONFIG_MMC is not set
 CONFIG_SPI_FLASH=y