drivers: sensor: bmi270: Use bulk SPI reads

BMI270 supports bulk register reads, use them to make
trasnfers faster.
See the discussion on
https://github.com/zephyrproject-rtos/zephyr/commit/6cbb84c3eed97efa33375ae24d050abf49ee5b86#r104543405


Tested on an nrf52840 board connected to a bmi270 sensor via SPI.

Signed-off-by: Oleg Ryjkov <oryjkov@gmail.com>