Fix the clkdev API support for the ep93xx uart clocks.

The uarts available in the ep93xx have individual clock controls.
The current implementation assumes that the bootloader has enabled
the clocks before the kernel has booted. It also assumes that the
bootloader has set the UARTBAUD bit indicating that the uarts are
running off the 14.7456MHz external crystal.

This fixes both issues. It also allows the uart clocks to be stopped
when there are no users.
Tested-by: Matthias Kaehlcke <matthias@kaehlcke.net>

Cc: Ryan Mallon <ryan@bluewatersys.com>
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>