In Thumb2 mode, the stack register r13 is deprecated if the
destination register is the program counter (r15). Similar to
head.S, head-nommu.S uses r13 to store the return address used
after configuring the CPU's CP15 register. However, since we do
not enable a MMU, there will be no address switch and it is
possible to use branch with link instruction to call
__after_proc_init.

Avoid using r13 completely by using bl to call __after_proc_init
and get rid of __secondary_switched.

Beside removing unnecessary complexity, this also fixes a
compiler warning when compiling a !MMU kernel:
Warning: Use of r13 as a source register is deprecated when r15
is the destination register.

Tested-?by: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Signed-off-by: NStefan Agner <stefan@agner.ch>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>