ARM: dts: palmetto: Request mux as per strapping configuration

Prevents the error:

 aspeed-smc 1e630000.spi: Error applying setting, reverse things back

The pinmux driver is only capable of modifying selected strapping bits
that make sense to change at runtime. The SPI strapping is not
currently defined modifiable, so "request" the mux as per how the board
is physically strapped.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>