cmd: Make CONFIG_CMD_FPGA depend on CONFIG_FPGA

cmd/Makefile has:

ifdef CONFIG_FPGA
obj-$(CONFIG_CMD_FPGA) += fpga.o
endif

which means that if CONFIG_FPGA is not set, CONFIG_CMD_FPGA silently
does nothing. Let's remove that Makefile conditional and instead express
this equivalent dependency in Kconfig, so a lot of redundant

 # CONFIG_CMD_FPGA is not set

can be removed from board defconfigs that don't actually have an FPGA.

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>