arm: mptimer: Remove WDT distinction

arm: mptimer: Remove WDT distinction

In QEMU emulation, there is no functional difference between the ARM mpcore
private timers and watchdogs. Removed all the distinction between the two from
arm_mptimer.c and converted it to be just the mptimer. a9mpcore and arm11mpcore
just instantiate the same mptimer object twice to get both timer and WDT.

If in the future we want to make the WDT functionally different then we can use
either QOM hierarchy to derive WDT from from mptimer, or we can add a property
"is-wdt" or some such.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>