README: davinci: Clarify when SPL is used and the target devices.

The documentation states that SPL is enabled in all config options
for the da850.  This incorrect, because devices booting from NOR
do not need the SPL to do the low level initializion because when
booting from NOR, the board is able to execute in place (XIP)

This also clarifies that SPL isn't only used for booting from SPI,
because it is also used for booting from MMC and NAND for those
devices supporting those boot options.

Signed-off-by: Adam Ford <aford173@gmail.com>