target-mips: add Unified Hosting Interface (UHI) support

target-mips: add Unified Hosting Interface (UHI) support

Add UHI semihosting support for MIPS. QEMU run with "-semihosting" option
will alter the behaviour of SDBBP 1 instruction -- UHI operation will be
called instead of generating a debug exception.

Also tweak Malta's pseudo-bootloader. On CPU reset the $4 register is set
to -1 if semihosting arguments are passed to indicate that the UHI
operations should be used to obtain input arguments.

Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>