x86/tools/inmates: Account for 32-bit PM timers

Some systems may use 32-bit PM timer (as defined by TMR_VAL_EXT feature
flag in FADT), however pm_timer_read() assumes it is always 24-bit. Where
this assumption is wrong, return value becomes incorrect, and the error
grows over time leading to obscure bugs, including lockups in the hypervisor.

To fix this, TMR_VAL_EXT is made part of platform config and is passed to
inmates in the communication region. Config generator was also adapted
to parse FADT to get TMR_VAL_EXT value for target system. pm_timer_init()
function was also introduced to the inmates framework to the overflow value.

Signed-off-by: Valentine Sinitsyn <valentine.sinitsyn@gmail.com>
[Jan: mark jailhouse_comm_region as packed]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>