clocksource/drivers/tcb_clksrc: Stop depending on atmel_tclib

atmel_tclib is probed too late in the boot process to be able to use the
TCB as the boot clocksource. This is an issue for SoCs without the PIT
(sams70, samv70 and samv71 families) as they simply currently can't boot.

Get rid of the atmel_tclib dependency and probe everything on our own using
the correct device tree binding.

This also allows getting rid of ATMEL_TCB_CLKSRC_BLOCK and makes the driver
a bit more flexible as the TCB is not hardcoded in the kernel anymore.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>