nxp: Migrate CONFIG_DDR_CLK_FREQ to Kconfig

As this symbol can either be a fixed value or the function
get_board_ddr_clk, migration is tricky.  Introduce a choice of DYNAMIC
or STATIC_DDR_CLK_FREQ.  If DYNAMIC, we continue to use the board
defined get_board_ddr_clk function.  If STATIC, set CONFIG_DDR_CLK_FREQ
to that value and now include/clock_legacy.h contains the function
prototype or defines get_board_ddr_clk() to that static value.  Update
callers to test for DYNAMIC or STATIC.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/ls2088ardb_tfa_SECURE_BOOT_defconfig b/configs/ls2088ardb_tfa_SECURE_BOOT_defconfig
index 21ad48a..e48ef7e 100644
--- a/configs/ls2088ardb_tfa_SECURE_BOOT_defconfig
+++ b/configs/ls2088ardb_tfa_SECURE_BOOT_defconfig
@@ -42,6 +42,7 @@
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_DM=y
 CONFIG_SATA_CEVA=y
+CONFIG_DDR_CLK_FREQ=133333333
 CONFIG_DM_I2C=y
 CONFIG_I2C_SET_DEFAULT_BUS_NUM=y
 CONFIG_I2C_MUX=y