arm: sc5xx: Remove SC5XX_LOADADDR

arm: sc5xx: Remove SC5XX_LOADADDR

Remove the SC5XX_LOADADDR Kconfig option, replace its users with
CONFIG_SYS_LOAD_ADDR, and update the ADI boot environment to use
`loadaddr`.

SC5XX_LOADADDR was an ADI-specific duplicate of standard U-Boot
load address handling. U-Boot already uses CONFIG_SYS_LOAD_ADDR
for the default load address and `loadaddr` for boot commands,
so keeping separate SC5XX-specific names is redundant.

Signed-off-by: Ozan Durgut <ozan.durgut@analog.com>
Signed-off-by: Caleb Ethridge <caleb.ethridge@analog.com>