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>