clocksource/drivers/timer-imx-tpm: Specify clock name for timer-of

i.MX TPM needs "ipg" clock for register access and "per" clock for
timer function, the driver gets "ipg" clock by searching the clock
name, but timer-of initialization will get first clock in device
tree TPM node since no clock name specified in of_clk, that means
the "per" clock MUST be the first clock entry in device tree TPM
node, this patch specifies clock name for of_clk to avoid this
restriction, it makes TPM driver work properly with different sequence
of clock entries in device tree TPM node.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>