armv8/ls2085a: Fix generic timer clock source

The timer clock is system clock divided by 4, not fixed 12MHz.
This is common to the SoC, not board specific. Primary core is
fixed when u-boot still runs in board_f. Secondary cores are
fixed by reading a variable set by u-boot.

Signed-off-by: York Sun <yorksun@freescale.com>
CC: Mark Rutland <mark.rutland@arm.com>