drivers/timer/gecko_burtc: allow queries prior to timer initialization

Before BURTC is initialized, clock to its APB CSR file is stopped, so
any attempt to read BURTC regs results in BusFault. However,
many parts of Zephyr may call sys_clock getters even before sys_clock
driver itself is initialized. This change adds support for that:
sys_clock_elapsed() and sys_clock_cycle_get_32() simply return 0
if BURTC is not init yet.

Signed-off-by: Roman Dobrodii <rdobrodii@antmicro.com>