arm: exynos: adds ifdef for spi boot

arm: exynos: adds ifdef for spi boot

This patch fix following errors and warnings

spl_boot.c: In function 'exynos_spi_copy':
spl_boot.c:111:49: error: 'CONFIG_ENV_SPI_BASE' undeclared (first use in this function)
spl_boot.c:111:49: note: each undeclared identifier is reported only once for each function it appears in
spl_boot.c:142:2: error: 'SPI_FLASH_UBOOT_POS' undeclared (first use in this function)
spl_boot.c: In function 'copy_uboot_to_ram':
spl_boot.c:189:28: warning: unused variable 'param' [-Wunused-variable]
spl_boot.c: At top level:
spl_boot.c:107:13: warning: 'exynos_spi_copy' defined but not used [-Wunused-function]

Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>