zephyr: main: initialize the buttons
zephyr: main: initialize the buttons

Initialize the button states and enables the interrupts.

BRANCH=None
BUG=b:182398910
TEST=With the following CLs, tested the volume buttons working.

Change-Id: I4d5b6965915c1103093114c3b01acb965c619a37
Signed-off-by: Wai-Hong Tam <04b587fdf5845741a0c5a8b9cd59ca72d73ef8fc@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2792693
Reviewed-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec