serial: atmel_usart: Use fixed clock value in SPL version with DM_SERIAL

This patch adds an alterative SPL version of atmel_serial_enable_clk().
This enables the usage of this driver without full clock support (in
drivers and DT nodes). This saves some space in the SPL image.

Please note that this fixed clock support is only added to the SPL code
in the DM_SERIAL part of this file. All boards not using SPL & DM_SERIAL
should not be affected.

This patch also introduces CONFIG_SPL_UART_CLOCK for the fixed UART
input clock. It defaults to 132096000 for ARCH_AT91 but can be set to
a different value if needed.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Andreas Bießmann <andreas@biessmann.org>
Cc: Eugen Hristev <eugen.hristev@microchip.com>