build(rk3588): add build configs for rk3588

This is needed to support compiling new source file.
Also updated the default tf-a build configs for rk3588.

Signed-off-by: XiaoDong Huang <derrick.huang@rock-chips.com>
Change-Id: I9e4dd355623e26cc4b816ac6a98f0857f4a167e7
diff --git a/tf_config/rk3588-coreboot b/tf_config/rk3588-coreboot
new file mode 100644
index 0000000..bed4fea
--- /dev/null
+++ b/tf_config/rk3588-coreboot
@@ -0,0 +1,3 @@
+COREBOOT=1
+CROSS_COMPILE=aarch64-none-elf-
+PLAT=rk3588