arm: uniphier: use DM_TIMER of arm a9 global timer

All uniphier v7 SoCs have cortex-a9 and use cortex-a9 global timer
in a simple implementation. Now DM_TIMER of it is available
on 35751c7f ("timer: sti: convert sti-timer to arm a9 global timer"),
so let's switch to it.

The old driver reads the lower 32bits of counter field
and sets the prescaler as 50 with PERIPHCLK(=50MHz),
so the global timer works as a 32-bit 1MHz timer.

The DM_TIMER uses the whole 64bits with no prescaler,
so the global timer works as a 64-bit PERIPHCLK timer.

CONFIG_SYS_HZ_CLOCK is set as the default PERIPHCLK frequency,
if there is no 'clocks' property in devicetree.

Signed-off-by: Dai Okamura <okamura.dai@socionext.com>