powerpc: mpc8xx: Migrate to CONFIG_SYS_CLK_FREQ

8xx has CONFIG_8xx_GCLK_FREQ which is similar to
CONFIG_SYS_CLK_FREQ, and doesn't set CONFIG_SYS_CLK_FREQ.

Due to that, get_board_sys_clk() returns 0.

Remove CONFIG_8xx_GCLK_FREQ and use CONFIG_SYS_CLK_FREQ instead.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
diff --git a/configs/CMPC885_defconfig b/configs/CMPC885_defconfig
index 7dff6ff..484a81b 100644
--- a/configs/CMPC885_defconfig
+++ b/configs/CMPC885_defconfig
@@ -4,11 +4,11 @@
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="cmpc885"
 CONFIG_SYS_PROMPT="S3K> "
+CONFIG_SYS_CLK_FREQ=132000000
 CONFIG_ENV_ADDR=0x40004000
 CONFIG_MPC8xx=y
 CONFIG_TARGET_CMPC885=y
 CONFIG_MPC885=y
-CONFIG_8xx_GCLK_FREQ=132000000
 CONFIG_CMD_IMMAP=y
 CONFIG_SYS_SIUMCR=0x00620000
 CONFIG_SYS_SYPCR=0xFFFFFF8F