MIPS: start.S: simplify relocation offset calculation

MIPS: start.S: simplify relocation offset calculation

The current code uses four instructions and a
temporary register to calculate the relocation
offset and to adjust the gp register.

The relocation offset can be calculated directly
from the CONFIG_SYS_MONITOR_BASE constant and from
the destination address. The resulting offset can
be used to adjust the gp pointer.

This approach makes the code a bit simpler because
it needs two instructions only.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>