arm ti_mbox_attach: Write sysconfig to TI_MBOX_SYSCONFIG to request reset.

[jhb]

Summary

This variable was flagged as a set but unused warning as its value was
read from a register and then modified to set a bit
(TI_MBOX_SYSCONFIG_SOFTRST). After the variable is modified, the code
then loops waiting for the SOFTRST bit to go clear in the
TI_MBOX_SYSCONFIG register. Presumably merely reading from the
register does not request a reset as other places in the driver read
this register, so most likely the updated value of sysconfig setting
the reset bit is supposed to be written to the register to request a
reset before the polling loop that waits for the reset to finish.

[jhb]

Mostly seeking review to see if there is a real bug to be fixed here per my musings in the commit log.

[emaste]

Based on the loop looking for TI_MBOX_SYSCONFIG_SOFTRST to clear I suspect the intent was indeed to write to TI_MBOX_SYSCONFIG and the warning found a real bug. I'd almost be tempted to put the write in and ask for feedback on -current.

[jhb]

Write the register rather than marking the variable unused.

[emaste]

LGTM. Hopefully someone with appropriate hardware will test after this lands.