rockchip: rk3568-rock-3a: Fix SPI Flash alias

The commit 64f79f88 ("rockchip: rk3568-rock-3a: Enable boot from SPI
NOR flash") enabled SPI flash support by overriding the spi0 alias.

Correct this by adding a new spi4 alias in rk356x-u-boot.dtsi and
SF_DEFAULT_BUS=4 and SPL_DM_SEQ_ALIAS=y in defconfig. Also enabled
support for parsing and auto discovery of parameters, SFDP.

Fixes: 64f79f88 ("rockchip: rk3568-rock-3a: Enable boot from SPI NOR flash")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>