arm: Kirkwood: add SYSRSTn Duration Counter Support

This feature can be used to trigger special command "sysrstcmd" using
reset key long press event and environment variable "sysrstdelay" is set
(useful for reset to factory or manufacturing mode execution)

Kirkwood SoC implements a hardware-based SYSRSTn duration counter.
When SYSRSTn is asserted low, a SYSRSTn duration counter is running.
The counter value is stored in the SYSRSTn Length Counter Register
The counter is based on the 25-MHz reference clock (40ns)
It is a 29-bit counter, yielding a maximum counting duration of
2^29/25 MHz (21.4 seconds). When the counter reach its maximum value,
it remains at this value until counter reset is triggered by setting
bit 31 of KW_REG_SYSRST_CNT

Implementation:
Upon long reset assertion (> ${sysrstdelay} in secs) sysrstcmd will be
executed if pre-defined in environment variables.
This feature will be disabled if "sysrstdelay" variable is unset.

for-ex.
setenv sysrst_cmd "echo starting factory reset;
		   nand erase 0xa0000 0x20000;
		   echo finish ed sysrst command;"
will erase particular nand sector if triggered by this event

Signed-off-by: Prafulla Wadaskar <prafulla@marvell.com>