am437x_epos_evm: add SPL API, QSPI, and serial flash support

Enables support for SPI SPL, QSPI and Spansion serial flash device
on the EVM. Configures pin muxes for QSPI mode.

Signed-off-by: Sourav Poddar <sourav.poddar@ti.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>