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/ds414_defconfig b/configs/ds414_defconfig
index ce42f23..62cad53 100644
--- a/configs/ds414_defconfig
+++ b/configs/ds414_defconfig
@@ -39,6 +39,8 @@
 CONFIG_CMD_UBI=y
 CONFIG_ISO_PARTITION=y
 CONFIG_DEFAULT_DEVICE_TREE="armada-xp-synology-ds414"
+CONFIG_USE_ENV_SPI_MAX_HZ=y
+CONFIG_ENV_SPI_MAX_HZ=50000000
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_SPL_OF_TRANSLATE=y
 # CONFIG_MMC is not set