Revert "ARM: uniphier: add weird workaround code for LD20"

Revert "ARM: uniphier: add weird workaround code for LD20"

This reverts commit 45f41c134baf5ff1bbf59d33027f6c79884fa4d9.

This weird workaround was the best I came up with at that time
to boot U-Boot from TF-A.

I noticed U-Boot successfully boots on LD20 (i.e. CA72 CPU) by using
the latest TF-A.

Specifically, since the following TF-A commit, U-Boot runs at EL2
instead of EL1, and this issue went away as a side-effect.

|commit f998a052fd94ea082833109f25b94ed5bfa24e8b
|Author: Masahiro Yamada <yamada.masahiro@socionext.com>
|Date:   Thu Jul 25 10:57:38 2019 +0900
|
|    uniphier: run BL33 at EL2
|
|    All the SoCs in 64-bit UniPhier SoC family support EL2.
|
|    Just hard-code MODE_EL2 instead of using el_implemented() helper.
|
|    Change-Id: I7ab48002c5205bc8c013e1b46313b57d6c431db0
|    Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

However, if I reverted that, this problem would come back, presumably
because some EL1 code in U-Boot triggers this issue.

Now that commit f8ddd8cbb513 ("arm64: issue ISB after updating system
registers") fixed this issue properly, this weird workaround is no
longer needed irrespective of the exception level at which U-Boot runs.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>