cpu/arm/k60: implement generic SPI bus driver.

 - Only Master mode is supported for now.
 - Create SPI bus driver API.
 - SPI bus hardware driver in spi.c
 - Configuration is in config-board.h and spi-config.c

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
platform/mulle: Flash use spi driver.

 - Uses new SPI driver in cpu/arm/k60
 - Remove unnecessary disabling of interrupts in xmem.

Flash memory access is already protected inside flash.c by
spi_acquire_bus/spi_release_bus.

Interrupts must not be disabled when calling spi_transfer_blocking
since that will wait for an IRQ driven transfer to finish.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
platform/mulle: Use spi driver for lis3dh.

LIS3DH is now using SPI driver in cpu/arm/k60.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
[SQUASH ME] Mulle flash: cleanup, remove unused cruft.
[SQUASH ME] platform/mulle: Update flash driver after comments from @scrizt.
[SQUASH ME] platform/mulle: Remove lis3dh_arch_init()