fpga: lattice-sysconfig-spi: Add explicit include for of.h

With linux/acpi.h (in spi/spi.h) no longer implicitly including of.h,
add an explicit include of of.h to fix the following errors:

drivers/fpga/lattice-sysconfig-spi.c:146:35: error: implicit declaration of function 'of_match_ptr' [-Werror=implicit-function-declaration]

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>