OMAP4: ST: Disabling UART if ldisc not installed.

OMAP4: ST: Disabling UART if ldisc not installed.

In cases when there is no line discipline to be installed UART chip must be
disabled.

Rationale: if ldisc is not installed so it can't be uninstalled.
Thus UART chip still enabled and we have wakelock for it
(see plat_uart_enable() function in both blaze and 44xx-tablet board files).
In that case we can't go in suspend mode.

Motivation: when using busybox ramdisk (e.g. for testing kernel w/o AFS) we
have no ldisc that causes impossibility to go to suspend mode.

Change-Id: I6fac0b9411ed765e8e598328b89a41a50b5fbfea
Signed-off-by: Semen Protsenko <semen.protsenko@ti.com>