arm: mvebu: Remove A38x BOOT_FROM_UART_ALT 0x3f constant

A385 BootROM treats strapping configuration 0x3f as invalid. When booting
fails (e.g. because of invalid configuration) then BootROM fallbacks to
UART booting.

Detecting BootROM fallback to UART booting is implemented in U-Boot since
commit 2fd42840 ("ARM: mach-mvebu: handle fall-back to UART boot").

So there is no need to define BOOT_FROM_UART_ALT constant and special
handling for it anymore, remove it.

This change effectively revers commit f3a88e2c ("arm: mvebu: fix boot
from UART on ClearFog Base").

Signed-off-by: Pali Rohár <pali@kernel.org>
Tested-by: Tony Dinh <mibodhi@gmail.com>
Tested-by: Martin Rowe <martin.p.rowe@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>