ARM: dts: am4372: fix clock source for arm twd and global timers

ARM TWD and Global timer are clocked by PERIPHCLK which is MPU_CLK/2.
But now they are clocked by dpll_mpu_m2_ck == MPU_CLK and, as result.
Timekeeping core misbehaves. For example, execution of command
"sleep 5" will take 10 sec instead of 5.

Hence, fix it by adding mpu_periphclk ("fixed-factor-clock") and use
it for clocking ARM TWD and Global timer (same way as on OMAP4).

Cc: Tony Lindgren <tony@atomide.com>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Tero Kristo <t-kristo@ti.com>
Fixes:commit 8cbd4c2f ("arm: boot: dts: am4372: add ARM timers and SCU nodes")
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>