ARM: dts: am335x-boneblack: move fixed regulator to board level

3.3V fixed regulator does not belong to TPS node - as a result
the fixed regulator is never probed and MMC is continually deferred
due to lack of regulator.

Move the fixed regulator to be at root of platform.

Cc: Joel Fernandes <joelf@ti.com>
Cc: Sekhar Nori <nsekhar@ti.com>
Cc: Koen Kooi <koen@dominion.thruhere.net>
Signed-off-by: Nishanth Menon <nm@ti.com>
Tested-by: Felipe Balbi <balbi@ti.com>
Tested-by: Balaji T K <balajitk@ti.com>
Signed-off-by: Benoit Cousson <bcousson@baylibre.com>