boards: arm: b_l072z_lrwan1: Fix SPI1 pinmux config

SPI1, which is used by the LoRa radio, has the following
configuration:

  SCK: PB_3
  MISO: PA_6
  MOSI: PA_7
  NSS: PA_15

Update the pinmux configuration and documentation to reflect this.

Signed-off-by: Andreas Sandberg <andreas@sandberg.pp.se>