cpu/ti/am335x: Move from cpu to soc in tree

The AM335X is a SoC, so should be in the soc tree.

This moves all the existing am335x code to soc/ and updates any
references. It also adds a soc.c file as required for the ramstage.

Change-Id: Ic1ccb0e9b9c24a8b211b723b5f4cc26cdd0eaaab
Signed-off-by: Sam Lewis <sam.vr.lewis@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44378
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

MAINTAINERS

@@ -550,7 +550,7 @@ F:	src/vendorcode/intel/fsp/fsp2_0/copperlake_sp
 ORPHANED ARM SOCS
 S:	Orphaned
 F:	src/cpu/armltd/
-F:	src/cpu/ti/
+F:	src/soc/ti/
 F:	src/soc/qualcomm/
 F:	src/soc/samsung/
 F:	util/exynos/