Initial PINE A64-LTS support

Here's my WIP branch for the PINE A64-LTS, mainline u-boot crashes before starting the kernel:

U-Boot SPL 2018.05 (Sep 08 2018 - 15:35:14 +0000)
DRAM: 0 MiB
### ERROR ### Please RESET the board ###

This branch also depends on https://gitlab.com/postmarketOS/pmbootstrap/merge_requests/1693

main/linux-postmarketos-allwinner/APKBUILD

@@ -3,7 +3,7 @@ _config="config-${_flavor}.${CARCH}"
 
 pkgname=linux-${_flavor}
 
-pkgver=4.18
+pkgver=4.18.0
 pkgrel=0
 
 arch="aarch64"