ARM: zynq: Fix sparse warnings in slcr.c

ARM: zynq: Fix sparse warnings in slcr.c

Warnings:
arch/arm/cpu/armv7/zynq/slcr.c:21:6: warning: symbol 'zynq_slcr_lock' was not declared. Should it be static?
arch/arm/cpu/armv7/zynq/slcr.c:27:6: warning: symbol 'zynq_slcr_unlock' was not declared. Should it be static?
arch/arm/cpu/armv7/zynq/slcr.c:34:6: warning: symbol 'zynq_slcr_cpu_reset' was not declared. Should it be static?
arch/arm/cpu/armv7/zynq/slcr.c:54:6: warning: symbol 'zynq_slcr_gem_clk_setup' was not declared. Should it be static?
arch/arm/cpu/armv7/zynq/slcr.c:81:6: warning: symbol 'zynq_slcr_devcfg_disable' was not declared. Should it be static?
arch/arm/cpu/armv7/zynq/slcr.c:94:6: warning: symbol 'zynq_slcr_devcfg_enable' was not declared. Should it be static?
arch/arm/cpu/armv7/zynq/slcr.c:107:5: warning: symbol 'zynq_slcr_get_boot_mode' was not declared. Should it be static?
arch/arm/cpu/armv7/zynq/slcr.c:113:5: warning: symbol 'zynq_slcr_get_idcode' was not declared. Should it be static?

Signed-off-by: Michal Simek <michal.simek@xilinx.com>