rtems: Fix sp2038 test.

Avoid using newlib's gmtime_r call which fails with a max signed int.
Add an RTEMS specific version for 1/1/1988 to 31/12/2100.

Update sp2038 to test every day from 1/1/1988 to 31/12/2100. Only days
need be tested as the code splits the seconds based on days.
bsps/zynq: Add BSP_ARM_A9MPCORE_UARTCLK to set the UART clock rate.

This value can be found the xparameters.h file generated by the
Xilinx tools.
bsp/zynq: Add BSP_ZYNQ_RAM_LENGTH to allow a user to override the RAM length.

The Zynq BSPs can be used with varations of hardware such as memory size.
This option lets you set a length.