clocksource/drivers/timer-gxp: Add HPE GXP Timer

Add support for the HPE GXP SOC timer. The GXP supports several different
kinds of timers but for the purpose of this driver there is only support
for the General Timer. The timer has a 1us resolution and is 32 bits. The
timer also creates a child watchdog device as the register region is the
same.

Signed-off-by: Nick Hawkins <nick.hawkins@hpe.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>