dra7xx_evm: Add QSPI_4 support, qspiboot build target

We previously only supported QSPI_1 (single) support.  Add QSPI_4 (quad)
read support as well.  This means we can be given one of two boot device
values, but don't care which it is, so perform a fixup on the QSPI_4
value.  We add a qspiboot build target to better show how you would use
QSPI as a boot device in deployment.  When we boot from QSPI, we can
check the environment for 'boot_os' to control Falcon Mode.

Signed-off-by: Sourav Poddar <sourav.poddar@ti.com>
Signed-off-by: Tom Rini <trini@ti.com>