emul: Make emulator parameters consistently named

Some paramaters in prior emulators have historically referred to instances
of struct emul as "emulator" or "emul". The proper parameter name for this
type is "target" as it designates the struct emul instance is the emul
bus (e.g. emul_i2c) controller's target peripheral for dispatch.

Do a small refactor renaming some of these parameters to "target".

TEST=twister on accel, espi, and eeprom drivers tests

Signed-off-by: Aaron Massey <aaronmassey@google.com>