hw/avr/atmega: Fix wrong initial value of stack pointer

hw/avr/atmega: Fix wrong initial value of stack pointer

The current implementation initializes the stack pointer of AVR devices
to 0. Although older AVR devices used to be like that, newer ones set
it to RAMEND.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1525
Signed-off-by: Gihun Nam <gihun.nam@outlook.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <PH0P222MB0010877445B594724D40C924DEBDA@PH0P222MB0010.NAMP222.PROD.OUTLOOK.COM>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
(cherry picked from commit 235948bf53860a1e2df5134eae7b0a30a971a124)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>