api: Rework menu, and make it depend on CC_IS_GCC

We can only use the old U-Boot API for standalone applications when
building U-Boot with GCC as it relies upon the "gd is a register" trick
that only GCC supports. Further, rework the rest of the options so that
they are in the API menu and only visible if API support is enabled.

Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

api/Kconfig

-menu "API"
-
 config API
 	bool "Enable U-Boot API"
+	depends on CC_IS_GCC
 	help
 	  This option enables the U-Boot API. See api/README for more information.
 
+menu "API"
+	depends on API
+
 config SYS_MMC_MAX_DEVICE
 	int  "Maximum number of MMC devices exposed via the API"
-	depends on API
 	default 1
 
 config EXAMPLES