MLK-17491-20 clocksource: imx-tpm: code cleanup and improvement

MLK-17491-20 clocksource: imx-tpm: code cleanup and improvement

1) replace magic number with macros
2) add more code comments
3) replace setup_irq with request_irq as no need use setup_irq
4) replace __raw_readl which has endian issue with readl
(so far endian issue not exist on IMX, but better write more robust code
in advance)
5) remove the last meaningless timer_base read

No function level change.

Cc: Shenwei Wang <shenwei.wang@nxp.com>
Reviewed-by: Bai Ping <ping.bai@nxp.com>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>