omap1: Fix redundant UARTs pin muxing that can break other hardware support

Commit 15ac408e

 removed enabled_uart
and OMAP_TAG_UART. This works for mach-omap2, but causes issues on
mach-omap1 for some boards as the mach-omap1 serial.c was muxing
pins based on the enabled_uart flag for 15xx.

Fix this by muxing pins in board-*.c files for the 15xx boards for
the uart ports that had enabled_uart flag set before the commit
above.

Tested on Amsdtrad Delta only.

Note that in the future we should add support for powering down
the uarts with a timer like mach-omap2/serial.c does. Otherwise
the enabled uarts will be blocking retention-while-idle.

Signed-off-by: Janusz Krzysztofik <jkrzyszt@tis.icnet.pl>
Signed-off-by: Tony Lindgren <tony@atomide.com>