rtc: brcmstb-waketimer: introduce WKTMR_ALARM_EVENT flag

This commit defines bit 0 as the bit of interest within the
BRCMSTB_WKTMR_EVENT register to make the implementation more
readable.

Signed-off-by: Doug Berger <opendmb@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20230120190147.718976-2-opendmb@gmail.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>