imx8: add rom api wrappers

imx8: add rom api wrappers

The ROM API is thoroughly undocumented, but apparently passing the xor
of the real arguments as an extra argument is required [1]. Also, we
need to do the "save gd/restore gd" dance. These are both error-prone,
and lead to a lot of code duplication.

Since both imx8m[np] and imx8ulp SOCs have this, add a separate
translation unit which is included precisely when the new
CONFIG_IMX8_ROMAPI symbol is set, which provide convenience wrappers
that take care of computing the xor value as well as doing the gd
dance, and that thus have a more intuitive API. Subsequent patches
will make use of these to reduce boilerplate.

[1] One wonders, for example, if the check is only applied to the
lower 32 bits, or if we're implicitly relying on all 64-bit pointer
values we're passing effectively have 0 in the upper 32 bits.

Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>