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/arch/powerpc/cpu/mpc8xx/speed.c b/arch/powerpc/cpu/mpc8xx/speed.c
index ad3d3f9..1a882a3 100644
--- a/arch/powerpc/cpu/mpc8xx/speed.c
+++ b/arch/powerpc/cpu/mpc8xx/speed.c
@@ -14,7 +14,7 @@
 DECLARE_GLOBAL_DATA_PTR;
 
 /*
- * get_clocks() fills in gd->cpu_clock depending on CONFIG_8xx_GCLK_FREQ
+ * get_clocks() fills in gd->cpu_clk depending on CONFIG_SYS_CLK_FREQ
  */
 int get_clocks(void)
 {
@@ -28,7 +28,7 @@
 	 * (For example, the cogent CMA286-60 CPU module has no
 	 * separate oscillator for PITRTCLK)
 	 */
-	gd->cpu_clk = CONFIG_8xx_GCLK_FREQ;
+	gd->cpu_clk = CONFIG_SYS_CLK_FREQ;
 
 	if ((sccr & SCCR_EBDF11) == 0) {
 		/* No Bus Divider active */