Enable buffered-mode for accel/gyro sensors

During asm330lhh's driver initialization, there is a kernel message
which indicates failure of mapping irq like following:

irq: type mismatch, failed to map hwirq-39 for interrupt-controller@f1010000!

This is because of wrong statement in agl-refhw-common.dtsi and results
disabling buffered-mode of asm330lhh. With this change, the irq can be
correctly mapped and buffered-mode is enabled, i.e. buffer directory
comes up like following:

/sys/devices/platform/soc/e66d0000.i2c/i2c-3/3-006a/iio:device<n>/buffer

* Updates from Scott Murray:
- Changes merged into the main reference hardware patch as
  discussed in Gerrit.
- st,drdy-int-pin declaration added to mark int1 pin as explicitly
  used, matching other users of the driver.
- i2c3 bus speed increased to 400 KHz.  The ASM330LHH is the only
  device on it and is 400 KHz capable, which testing bears out.
  If any of the higher ODRs will be used in buffered mode, this
  seems desirable.

BUG-AGL: SPEC-4098

Signed-off-by: Hiroyuki Ishii <ishii.hiroyuki002@jp.panasonic.com>
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I598b1195a20d9e4611d2e505779ebeebfcbd8757