arm64 has no __naked__ attribute and the compiler adds a function prologue
for saving x29 and x30 to the stack for all C functions. This includes
functions defined using the ENTRY_FUNCTION macro. Therefore, the stack
needs to be setup before entering a C function, which is not possible if
the entry is a C function.

Provide a macro to implement the entry in assembly to be able to setup the
stack before entering the low level entry function.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>