spi: Add dummy definitions for ACPI lookup functions

Merge series from Richard Fitzgerald <rf@opensource.cirrus.com>:

Provide empty versions of acpi_spi_count_resources(),
acpi_spi_device_alloc() and acpi_spi_find_controller_by_adev()
if the real functions are not being built.

This commit fixes two problems with the original definitions:

1) There wasn't an empty version of these functions
2) The #if only depended on CONFIG_ACPI. But the functions are implemented
   in the core spi.c so CONFIG_SPI_MASTER must also be enabled for the real
   functions to exist.